Now, there are different accounts on how Moscow came to be some kind of centre for The Russias, but there is basically one account on what happened at the ocnversion of Kiev. And that its immediate result was Sviataja Rus' or Sv'ataja Rus' depending on your preference for spelling the palatal element of a previous consonant before the a*.
There seems to be a disagreement on how originally Russian the Muscovites are. The story I had been told in academia in Sweden was basically this: Novgorod was a Hinterland of Kiev, later Suzdal and Moscow was a Hinterland of Novgorod. Even later than that it led the liberation from the Golden Horde.
If you do not know what Hinterland means, imagine the Highlands in comparison with the Lowlands in 1500, or Kentucky as compared with Virginia in 1800.
Note that this neither refutes that Muscovites have one part of the heritage, nor confirms that they should have the major part of it.
You see, while Kievan Rus' was free, Novgorod was at first like a minor colony to it. Then it seem Novgorod got conquered by the Golden Horde and Kiev (with so many other towns in Rus') by Lithuania, when it was in a very expansive mood. Gedimynas is unparalleled as a conqueror in Lithuanian history. One or two generations after him, the thitherto Pagan gradduke of Lithuania married, after baptism, the Queen of Poland. So, the capital of Kiev was as much Krakow, ultimately, as the capital of Novgorod was something like Ulan Bator, or whatever.
Then Czars of Moscow start to make war with Poland. Kiev is torn from Poland, nevertheless Ukraine for some time resists the domination of Muscovy. By then Muscovy had been for some time, at least seemingly in practise, independent of the Golden Horde.
Which is more purer Russian? Certainly Ukraine. Ukraine is Russia, original version, Novgorod perhaps and Rostov - Suzdal - Moscow certrainly, you get Russians coming there and you get Finns living there before becoming Russians. This was, however, already happening in the time of St Vladimir the Great.
So what are the other criteria?
After they had separated Kievan Rus' retained political independence a bit longer, Alexander Nevski had abaondoned it to the Golden Horde before 1300, but Kievan Rus' to "Vyeliki Knyaz Kgidimin", in other words Gedimynas, after 1300.
Who regained political independence from either Pagans or anyone first after that?
From Pagans - Kievan Rus'. The inheritors of Gedimynas became Christian very soon, but the Golden Horde was a yoke thrown off later.
These internal struggles allowed the northern vassal state of Muscovy to rid itself of the "Tatar Yoke" at the Great stand on the Ugra river in 1480.
So, it was 1480 for Muscovy, but as to no longer depending on Pagans, it was a Century earlier for Ukraine:
Jogaila, later Władysław II Jagiełło (c. 1351/1362 – 1 June 1434) was Grand Duke of Lithuania (1377–1434), King of Poland (1386–1399) alongside his wife Jadwiga, and then sole King of Poland. He ruled in Lithuania from 1377. In 1386 in Kraków he was baptized as Władysław, married the young Queen Jadwiga, and was crowned King of Poland as Władysław II Jagiełło. In 1387 he converted Lithuania to Christianity.
On the other hand, while Ukraine ceased to depend on a Pagan in 1387, it remained a dependent on someone, first Poland, then Russia, up to the Russian Revolution, excepting brief spells of unsuccessful independence under Mazepa, the ally of Charles XII and the enemy of Peter the Great.
Now, what about religion?
When St Vladimir the Great converted and was baptised, he got his bishops from Constantinople, but from a Constantinople in Communion with Rome. Ukraine is divided if the Communions with Rome or Constantinople matter most. Muscovy was, as soon as showing any independence of Kiev, probably already very Anti-Roman.
What about language?
Is Ukrainean older or younger than Great Russian? BOTH are actually as written languages centuries younger than the earliest language written there. Slavonic - both Church Liturgic and Secular usages - had been encoded as to spelling, words, morphology, syntax, when a Missionary from Thessaloniki studied a nearby Bulgarian dialect in order to make his Mission in Moravia, which later was anyway Christianised the Latin way. That was the language officially written in Kiev as well as Suzdal when Suzdal was still a province of Kiev. Or a province of Novogorod which was a province of Kiev.
Probably the Russian language from back then was the same in both Kiev and Suzdal, or roughly so. But since it was hidden by Slavonic, I am not sure we know when it starts to diversify. For my part, I do not know when earliest traces of either start showing up in Slavonic texts. Of course, when a Slavonic text spells Gedimynas like Kgidimin, this might indicate that G was pronounced like in Ukrainean the H. A real and hard G was hard to come by. Was this an innovation which did not reach to Suzdal? Or an older state, and had Suzdal already a real G? I do not know, and I do not know if anyone does.**
In cities like Kiev and Lvov, you have names which have diversified. Ukraine keeps the soft V at the end, Moscow has for centuries kept it in spelling, but pronounced V as F at the end of words. On the other hand the vowels are better preserved in Moscow, since ie has become yi in Ukraine (Kyiv) and o has become i (Lviv). This latter point brings us to how come they say Ivan in Moscow? In Serbia they say Yovan, logically Moscow would say Yovan as well. It is in Ukraine that Yovan becomes Yivan = Ivan. So, Great Russian, for that one name, has the Ukrainean form.
So, linguistically the jungle is more or less inextricable which of the two shall count as "older" or "younger" brother. Religiously, it depends on which side you take in the 1054 conflict. Political independence as well as origins of Ruthenity, Ukraine takes a pretty clear precedence at first, but a disputed one later, depending on whether you consider the Polish Sovereignty as comparable to the Russian one or to that of the Golden Horde - which again depends on your religion.
To an Orthodox claiming Alexander Nevski did right to resist the Teutonic Order, I would suggest that: so did the Poles and Lithuanians, in 1410.
Any real Pope° should, as soon as possible, in union with all bishops in the world, consecrate Russia to the Immaculate Heart of Mary. And, I might suggest, leave it to Her, which of the two major Russias she means to have whatever kind of precedence over the other and perhaps over the third, the Bielorussians. Or if they could all come together as equals. Or whether, by now, it would mean Ukraine rather than Moscow and Minsk.***
Hans Georg Lundahl
Bpi, Georges Pompidou/Paris
St Dominic of Guzmán
4 - VIII - 2014
* You could theoretically also spell it Svyataya Rusy - but Rus'/Rusy is one syllable whichever way you spell it. It is yod as a slight glide, not as a full even unaccented vowel. But the spelling I gave is much more Slavonic, at least to judge by the use of Polish/Latin alphabet.
** As in: I have not yet checked whether this question is resolved or debated. A professional Slavonic Linguist would obviously know.
*** Minsk is Capital of Bielorussia. "Ukraine rather than" = Ukraine alone. The consecration asked for in Fatima has been very long protracted.
° The problem is, I do not think the one most widely recognised is the true one. Check with Pope Michael.