I perfer armor over another ftr for Russia.
IMO, you shouldn’t have too much problem trading, you really don’t have to trade Belo (if the choice is Belo vs. Ukr/Kar) or Kar (if UK can do it for you), and worst case you can always send 2 inf vs. 1 inf (retreat if go bad), or 3 inf vs. 1 inf on the territory that you want to conquer but it won’t kill you if you have to retreat, again usually Belo or Kar.
It is foolish to think that Russia will just randomly trade with armor and leave 1-2 exposed, however, if Russia has 10, 12, 14+ they can certainly expose their art since they won’t help on Def (might as well use on Off), and then maybe you can afford to throw an armor out there later (rd 4 or later) since UK and US should be arriving.
I do like to go ftr heavy with both UK and US, that is where I get the extra 4’s, let the other Allies pay the 10 while Russia focuses on the ground.
My concern about an early Russian ftr purchase is, yeah it may help to trade 3 ter a little more safely, but what if Germany isn’t interested in trading and moves heavy to Ukr. In order to push them out, not only are you down 1 unit (2 arm vs. 1 ftr) but you’ll likely have to expose your 3 ftrs to AA fire, and should one get shot down you’re now down two units in the attack. A potentional winnable able battle could turn into a disaster real quick.
Now, regardless of whether the AA gun hits, this is a move (and a chance) I’ll likely make as Germany and force the Russian player to either concede Ukr to me or risk his Russian army and new 10 ipc purchase to AA-fire and a German counter attack early in the game, before the UK and US can really get going. Whereas if they had 2 armor I’d be more worried about a strafe and be more likely to just hold back. It may be a fine line between 2 aa shots vs. 3 and 2 more units vs. 1, so the German move may be inevitable, but as Russia I feel a bit more comfortable with the arm, and as Germany a bit more aggressive if Russia goes ftr.