So...is this a 55 Tall? Cuz normally a 55 is 48"x12"x18"
If it's a tall tank, it might not be a bad idea to use the 40 as a display if sticking with your existing lighting is a major concern.
However...what are your intentions for the display tank? Fish only? FOWLR
If reef, are you wanting a few easy corals, or you wanna go all out?
If you wanna go all out, you'll probably want metal halide lighting, so you may as well go with the 55...but that's just my take.