I've had the P229 since 1994 and just very recently got an M&P 40c. I love my Sig and so far very much like my M&P. For range time, I would likely opt for my Sig as I shoot it lights out. My M&P is a compact though, so of course it is not as fun to shoot. I imagine the full size is very nice. Still, I have a hard time assuming it would be as nice as the Sig.
For carry, the M&P is lighter which is a huge variable as to how much carry time a gun will get.
For triggers, although the old Sig trigger is nice, it has a loooong reset and of course is DA/SA. M&P's can have gritty triggers out of the box, but they smoothen out. If you want, you can always install an APEX.
Value...That is a tougher one than you might know. Although Sigs are probably worth more, I have been coming upon more than a few for very reasonable prices. As per Rellascout, I believe that the value of many older Sigs being sold for very low prices are probably the best pistol deals going these days. Funny enough though, P228's are much harder to find at great prices versus P229's.
Bottom line, if you are not against polymer framed autos for carry, you should consider the usefulness of the M&P. But if you just have to have a P229, then make the trade and start saving your pennies for your next polymer-framed (light-weight) gun that you will carry in the future. Sigs are worth having...I'm just not sure that they are better for CCW than an M&P or whatever comparably light-weight, polymer-framed pistol.