Tech rolls are a gamble, but like any gamble you can turn out really big. Germany never really has the time or cash to go Tech, but Japan is another matter. I’ve seen games won simple because Japan got Heavy Bombers or Ind Tech.
About the tank/inf ratio, let me give you a little math.
If you buy 3 tanks (15) you get 1.5 hits first round.
If you buy 6 inf, you get 1 hit first round.
But the difference is that with the 6 inf you can take twice the hits, thereby getting more hits in the long run. On defense Inf are even better obviously.
The 9 inf 1 tank if basically 4 inf for Africa, 5 for East Europe, and 1 tank for some extra offense.