It's easy to switch back and forth any time until you start upgrading items to mythical. Even then, you can upgrade your core, legs, etc. and still switch between mech types.
I'd just use whichever you can rank higher with and run the campaign faster with, then if you get a really good item or two for one of them, you can commit more.
It's also worth nothing that's there are 3 good heat drones. Finding one of them shouldn't take you too long, whereas, on the other hand, counting on finding a certain legendary item is a real long shot.