I started my handgun ownership with an HK USP Compact in 9mm, moved to a Glock 23 in .40, and now have the M&P .45 with the 4.5" barrel. (I've sold each previous gun to purchase the next one.) I owned the Glock the longest, but never truly liked the way it shot. Some of it was specific to Glock, but some of it, I believe, was specific to the .40 cartridge. I've read others report a similar experience as mine with the .40. Of course, it's quite popular and plenty of people love it/swear by it, so it's subjective. It was just never enjoyable to shoot. It may have been due to the more compact nature of the G23. I don't know.
When I took my M&P .45 to the range for the first time, it was hands-down the most pleasant shooting experience I'd ever had with a handgun. More accurate than anything else I'd shot, comfortable in the hands, and just a joy to use.
A friend was with me who was in the market for a new handgun; he rented a Springfield, and had recently fired a Glock, Ruger, and Sig - after shooting my M&P, he made his decision and purchased the M&P 9mm (4" barrel) the next day. He said the 9mm was just as enjoyable to fire as my .45 was but with a little less kick (and more capacity).
I am extremely happy with my 4.5" M&P .45. I am ok with 10 rounds of a larger caliber, I like the extra .5" barrel (both in looks and in feel), and I'm comfortable with it for personal protection etc. It gets my vote.
(I purchased mine apparently after S&W started using the shield triggers, because I absolutely love the trigger on mine.)