I would certainly recommend Gambit here.

For me Tandem feels more a like a novelty plane, guns are really fun, but the deeper you get into the game the more it underwhelms due to lack of power. It does give extra brobot exp which is it's one real benefit...if you are desperate for more brobot exp it is worth a run. It's a bit double edged though...more brobot exp means you need more coins. Eventually I find my brobot points far surpass my coins so I can't spend them all anyway.

It also gives an extra Overdrive which is very nice, but you really want to learn to only use those when you really need them, I'd rather not get into a habit of using one overdrive per fight just for the sake of it. End game you'll likely keep it benched unless a Trial/Team Up demands it.

Gambit however is a plane you still find useful at the end game, it has a good overdrive (just takes a bit of practice to get the best out of it) and a very good counter attack making you briefly invisible. It also heavily reduces damage from lasers, those nasty lasers that occasionally surprise and kill you. 

It's also one of just two planes (along with Phoenix) that can revive itself. Meaning you can afford to be more flexible with your brobot choices, many of us like carry one revive bot...but you don't have to with Gambit. (While you could have 2, or even 3 revives if you want - good for Maze missions!)

So yeah, I'd 100% pick Gambit here.