Both should work but XR-30 if it's within your budget. The IM-20 is pretty rectangular vs cubed which means the 2 LED pads on the XR-30 will allow you to mount lower than the XR-15 with more even coverage and less light spillage even though it's overkill.
I just want to make sure the XR15 is enough for the tank. I have all Sps. I rather spend less on the XR15 now since I am sure the G5 will come out soon.