That's tough. It's hard to pick as I do like both and I think they both work. I think the one with the first one (with the cold background) makes the mech pops a little bit more, but, like you, I also like the look that the warm background version gives. It's almost like they're fighting in a desert or a very dusty environment. And it gives it a bit of a vintage look, like an old photo that's a bit yellowed out, which I find interesting. I'm not sure if it's a good or a bad thing. I guess it depends on what you want to show.
But in the end, whichever you pick I think it'll work!