you'll want one that has an front end that can be elevated up to 15-20degrees to similate hills or low impact stair climbing. also, pay for quality... you don't want something that doesn't feel sturdy or doesn't run smoothly... and some are more anatomically correct than others. like others say- tried and true ones are at the gym. don't fall for sales pitches that say their niche is household markets... as my in-house personal trainer says, that's crap. he stands by these ones:
http://www.allellipticals.com/best-elliptical-machines.html
oh, btw, in 2006 I was enquiring on terb about the same thing... ellipticals. you can probably find that thread somewhere... though I don't think I started it.
anyways, I went with a Precor EFX 5.23 ... it doesn't have upper body movement, so I pump my arms on my own... i wished I would have went with the 5.33, but I kinda like having free hands as it forces me to keep my 'balance' up. the 5.23 I got was a home model, not commercial... but it was plenty sturdy enough. I'm over 240lbs. and use it almost daily for 4 years now... my log says I average 5.33 days per week! 1hour each time. No wear issues and still going strong. I've logged over 1100 hours on it. and the best treadmills aren't meant to last 1/2 that without a tread replacement and new bearings and bushings.
hammertm has some good threads on the use of the elliptical.. he lost alot of weight on one that he bought, and was a good source of advice for me when I was looking to purchase... it also helped me to keep motivated to use it by setting it up infront of the tv. I'd watch for 1 hour everymorning while on it.
oh, and you can find these at boomerang fitness if you want used.. or on craigslist... just try before you buy- and inspect very well if it is in storage. be weary of rust. around bearings, etc. depending on the model you choose. make sure the computer works well, the crossramp moves up and down w/o error and the resistance works. make sure you get the user manual too.
or buy new. lots of good promos now for 'new years resolution' stock reduction.
next, I want one of those Jacob's ladders you see on fattest loser show.
http://www.bigfitness.com/jalatoboex.html?AID=10273789&PID=3637149