Hi Matt,
I just ordered the S650 myself....went through the same dilemma. I think I could've gotten by just fine with the 450, although we do entertain groups quite a bit. My final decision was made, really, by the extra room the 650 provides for indirect cooking, at which my wife and I would like to both become more adept...
There aren't any Weber dealers close to me (at least not one that shows up on Weber's dealer locator), so I ordered online. The lowest OVERALL price I could find was through
www.shopperschoice.com Amazon will route you through them, as well. I highlighted "overall", because with the 650, they give you a Revco marinator free.
That I could find, only one site on the web could match SC.COM, (which was
www.thenewconnection.com - I think) but they didn't include the marinator. One site had free shipping, one had a lower price on the grill - they netted the same (excluding the marinator).
I may or may not keep the marinator, but they retail on the web for $199.00 or so, so I would imagine I could put it on the 'Bay for somewhere close to that. That makes the overall price better than any other I found. I think the 450 comes with a set of stainless grill tools, if I'm not mistaken.
Shopper's Choice was backordered for a couple of weeks when I ordered mine. I just got an e-mail saying they should be in stock any day now, so if that holds true, you shouldn't have to wait if you go that route...
Take care!
Doug
EDIT: As Paul says below, these certainly aren't "discounted" prices I'm referencing, just the best online prices I was able to find at the time I ordered.