I think they have different capacities and different collection pots (some are thermos type). I haven't heard of any performing poorly.
Sweet Maria's has a hard-to-find discount page that sometimes offers open box ones for a bit cheaper. I got one off there for the gf and it works great.
Edit:
Found it! No Technivorms right now.
http://www.sweetmarias.com/prod.sale-items.shtml