Just speaking about the trigger pull, I'd say yes it would be worth the extra money. But then again if you think you'd be one to do a trigger job on your gun, even if 6-9 months down the road then maybe you could get away with the $100 cheaper gun?
What most people who would want to know is, is it mainly just for target practice or is it for home defense (+P or +P+ rounds)? If you put that in your original post, people that know about both would be able to help you more.
But I really don't know anything specific about these two pistols though, so others will have to guide you better than I can. However two weeks ago I shot the S&W M&P9 full size and the M&P9C. They were great guns. I'm coming from a XDM9, previously a Glock 23C with the 3.5 lbs. trigger and I was really impressed with the M&P's.
I see that BudsGunShop (cash prices) has the SW9 for $371 and the SD9 for $386, of course maybe I'm looking at the wrong ones since they are so close in price.
If those are correct then I would without a doubt get the M&P9 though, since JetGuns has the M&P9 Full size for $439, that's with S/H and it's also if you pay with a credit card. Of course I have no idea where and how much you'll be paying for those two, I thought I'd just throw that out there. I'd rather wait an extra 1-2 months to save up for the M&P though, but that's just me. Even if you are going to buy from a local store and the M&P is another $180 I still think it'd be worth it. But then again they make and sell those handguns for a reason, they are cheaper and they still work. Hell, guns seem to be the only thing made these days that actually last, basically guns and cars that take beatings and keep going.
But with either of those, from what I've been reading since June 2010, S&W is really good with their warranty so both should do you well.
If you haven't shot them, or only one and it's been more than a week. I would highly recommend to go rent the SW9 and the SD9 or even the M&P. I'd post in forums that have a State and/or local section where people close to you might let you shoot it. Can't hurt to ask if someone is going to the range in 1-2 weeks or so in your area, might have to post to 3-5 forums though. I know I'd let you shoot mine for free if I had one, or any gun I owned if I saw a post asking and I was planning on going to the range, if I wasn't and they offered to pay for my range fees then I'd be all over it for sure. Just make sure to say you'll bring your own ammo (a given but say it nonetheless) or buy some there and offer to pay for a couple targets and let them shoot one of your guns.