Pedals

I finally have the opportunity to upgrade to clip pedals. I've been using cages for about a month now and the hubby is finally tired of hearing me bitc$ about them.

I'm looking at two pedals. The Shimano XT's (770's) and the XTR's. Both SPD's for mtn biking.

My question is what is the big difference between the two besides the price?

Also, do you know where I can get a good deal on either of these? I checked ebay for the XTR's (those are the one's I really want) and found them, with clips for $129 but I wonder if I can do better than that.

Thanks

Terie