Who is better?

Those are the two best options, so I see no point in including others. Vasilevsky is eh, Slim is trash, and Clark is worse. Sorry Gerdy.

Do you like going defensive or offensive? Then Tassigny or Bradley respectively.

In the middle?

Reserve is near useless, so we take that away.

For defense: 

Tassigny: You basically have a 30% chance (a more liberal guess, no, not politics) or a 40% chance (a more conservative guess, no, not politics) to NOT reduce damage for Tassigny. The mean would be a 35%, or you reduce damage 65% of the time. I like those odds.

Bradley: Your survivability is actually reduced by 24%, a skill known as commander screws that up.

For offense:

Tassigny offers a 12 damage boost when attacking demoralized enemies. Assuming this is on 2/3rds of the time (not that hard to activate it a majority of the time), Tassigny will get +8 damage.

Brad offers JF, which you will be on about 20% of the time, tops. +2 damage. IL is +18.864 (ET rounds down, fyi), so +18 damage +2 is +20. Clear win for Brad.

Total:

Tassigny has +65% defense and +8 offense.

Nullifying IL, however (it -s defense and +offense by 24%), so you get +0 defense and only +2 offense.

I think it is safe to say that for a generalist, Tassigny is better, but for a full offensive player who can make (what little) use of Brad's Reserve as possible, Brad will be better.

I would say it's Tassigny who takes the cake for me.