Does it have to be those two stones? Are you limited to a trade-in situation or are you open to other vendors?
I wouldn't go below excellent in cut (since cut makes the sparkle), but ideal is well... ideal. With an ideal cut stone, you will get so much light return from the top of the diamond that any color above an H/I will appear very very white. I personally wouldn't go past VS in clarity (but then again, I have eagle eyes). An ideal cut will look bigger than a poorer cut one, if that matters.
If you're open to having the 1.55 re-cut to about a 1.35, I would look for the best possible 1.35 to save yourself the trouble. There's no sense in paying for 1.55 carat weight, then paying to have the stone re-cut to certain specifications and remounted when you can just buy what you really want in the first place.
Check other vendors for a G/VS Ideal Cut in the biggest size within your range, and you will NOT regret it!