It really does depend on the situation. Need to hurry folks to a front a few turns away? Buy the mech inf and tank.
Landing on an island and have one transport? Buy a tank and inf. Best bang after suffering 1 casualty.
Defending and have only 9 bucks? Lets see
3 Infantry……
1 avg hit 1st round of defense, lets say you loose one…
.67 hits second round, loose another…
.33 hits last round, loose the last
Gives you 2 hits.
Tank and inf defending…
.5+.33=.83 hits first round, loose one…
.5 hits second round, loose one.
1.33 hits total
Over the course of this game the map is soooo big I love the speed of the tank. They also defend well in captured territories and on transports or where your build capacity is limited tanks gain importance.
Transport attack math
1 artillery 1 inf
.67 hits first round, loose inf
.33 hits second round, loose artillery.
avg 1 hit.
1 tank 1 inf
.5 + .33 = .83 hits 1st round, loose the inf (.33)
.5 second round, loose one
1.33 hits.
It DOES cost more to build the tank and inf though.
With Germany my infantry/art/mech/tank purchase rate is
6inf, 1 art, 1 mech, 1 tank. I LOVE the fast moving tanks. Sometimes I plop 3 tanks down in Lenningrad if there is a big attack pending the next turn.
Russia?
Probably 8 inf, 1 art, 1 mech. Sometimes I build a tank. Perhaps .5 tanks per turn lol.