jfinet,
I've got a P229 .40 S&W as well. After MUCH research, I decided on the M&P9c. It is SO MUCH easier to carry, and lighter. I have zero issues concealing it, even with only a t-shirt. I'd highly recommend the M&Pc as your summer carry weapon.
Personally, I opted for the 9 instead of the .40 because I didn't want to make any type of a sacrifice in capacity . . . so with my 9c, I actually GAINED capacity over my P229, while reducing overall size and weight. I believe that shot placement is more important in stopping a BG than a couple hundredths of an inch difference in bullet diameter, but you're welcome to hold whatever opinion you want!
Best wishes.