You can't really compare them like that, a 5" rhom is still a juvi and it's normal if it hides a lot, at least at first it will until it's settled in your tank. once it feel comfortable it will be more active. it also depends on each fish's personality, some could be crazy right off the bat and some can be skittish and takes a good amount of time until it comes out of its shell. same goes with ruby red spilo but at 7" its at adult size so it could act more bold than a small juvi rhom but again it depends on the personality of the fish. the question is which fish you like to look at more? what made you to get the xingu rhom over the ruby red spilo in the first place?
personally I would get a much larger rhom for a 75g if you don't want to grow one out yourself but that's just me.
where did you get rhom from anyway if you don't mind me asking.