Whichever I do, I'll most likely do the other in a different play-through, so right now I'm thinking about who else I want to try playing as. Might be fun to form the SU as Polish or Bohemian - Russia doesn't offer options like that. Bloodline is nice, but I already forged one and married in two others, and haven't done a legendary journey yet. So... it's currently leaning towards Russia.The Slavic Union and the Russian Empire are both rare in the game in that creating either one of them grants special game-technical benefits.
If you form the Slavic Union you will get a bloodline.
The holder of the Russian Empire has the ability to upgrade tribes to castles with only a tier 1 hillfort (instead of the usual tier 4 requirement). This is because its supposed to be a representation of the Russian tsardom come early and enable you to "colonize" eastward into Asia as the Russian Empire did.
I'd recommend deciding on which benefit is better for you and go for that one.
Dobrava of Kiev. I'd AAR it, except I have no screenshots from the first two rulers.Who did you start as?