I happen to have an XD45 Service (4") that I absolutely love. The 4" compact is on my list of to acquire firearms (hey... same ammo, mags and holsters, what's not to love).
I personally like the 45 for carry. That being said, I'm currently carrying a glock 19...and I surely don't feel under armed.
Is there a range near you where you can rent one of each and try it out. Personally, I think you'd be well suited with an XD in 9, 40 or 45ACP. I'd tend to shy away from the 45 GAP, because of cost and availability concerns.
IMHO, the 45 gap is a solution in search of a problem, esp. in the XD :)