How do you feel about power cables? I think either setup will work out fine. I had an XR30 on my IM 25-gallon lagoon and it was overkill. I’m currently running a pair of XR30’s and a single XR15 on my RedSea 750XXL.
If you’re going to have corals run right up to the sides of the tank the pair of XR15’s will afford you better coverage. You’ve also got some planned redundancy if one light were ever to fail.