It is true that the .40 has more of a snap and the .45 is a pusher. That is one of the reasons I like the .45. It is also the caliber I shoot more accurately with. If I were you I would not worry about it. Some like the .40 more because of the snap. It will recoil faster and get your sites back on target. I have never had a chance to shoot the .40 in an HK but I would assume it handles the round better than most. Worst .40 I have ever shot was a Glock 22. It had noticeably more recoil then my .45s. I am sure you will be happy with whatever you choose! There is no perfect round. Just be sure to practice!
I gotta agree with gunnut. I just got the USPc in .45 and I really like it. Previously I've shot .45 in a full size Kimber 1911. The USPc is by far softer shooting. The first time I shot the gun (last Sunday) I put one big, ragged hole in the target at 30'.
I have a Sig P229 in .40 and it's not bad. The recoil is very manageable, but a little snappier than either a 9mm or .45. I have also shot my wife's P99QA in .40 and it shoots well, except the trigger hurts my finger after a while (go figure.)
Sounds like you have a good plan - try out the guns you're interested in. I wouldn't go in fixed on one caliber.
I had the two you mention together for about five years ... recently sold the 45c, still own the 40c ... I get back on target with the quicker snap of the 40. The torqey roll of the 45c is slower for me. I get a much better grip on the frame of the 40c. A good shooting strong hand grip on the draw is a critical factor is recoil control and quickness back on target.
For those that ask why sell the 45c .... I just did not shoot it that much and sold it to aquire a XD Tac in 40 plus several extra mags. The 45c is a excellent gun. Plus I recently got a green framed 40 Comp Carry so all is happy in the HK family.
As other said ... go try the two for yourself. The grip size is dramtically different between the two. Please see my holster sale on a Blade Tech if you are in need.