Since both calibers have pretty much the same recoil Id go for the XD45 it has a higher magazine capacity too. the XD40 is 12+1 the XD45 is 13+1. I beleive a 45 has better incapacitating capabilities then the 40 which is a big plus if you ever had to use it in self defence and since the XD45 holds one more round over the XD40 I think the XD45 is deffinately the superior choice.

I have a XD45 tactical (5 inch barrel) I love it its very accurate and practical. at 25 yards in a standing 2 handed position I can put a full magazine into a paper plate, I cannot complain one bit. the xd line is an excellent choice though many glock buffs will disagree with me. you just cant go wrong with a XD no matter what caliber you choose.