Another factor here is multinational armies. I don’t know how common these will be, but perhaps a pattern might be:
1. Only France buys tanks, in a fairly large stack.
2. France makes a tank attack, breaking through the enemy lines to capture a tt, but having to take inf and art as casualties.
3. The UK moves in large numbers of inf and art to defend the area (and the vulnerable tanks) from an enemy counter-attack.
4. France has to move up its own stacks of inf and art to prepare for the next attack.
The problem with this is that you have to spend a whole turn in between bringing up fresh troops to support the tank breakthrough. Far from speeding up a drive towards the enemy capital, tanks actually slow it down.
Perhaps, if you’re camped outside the enemy capital, the one tt you need to win the game, it will pay to build tanks for the final push; but in general game play I think 2 inf will always be the better buy.