I'm contemplating buying one or the other. So, like an smart shopper when buying a big item, I'm doing my research! I always say that google does wonders =)
In doing my bit of research here, I have noticed that it really depends on your overall exercise goals when choosing between the two.
From reading several different sites and articles I'm seeing that both machines are good for an effective aerobic workout.
The major difference I'm seeing between the two is that ellipticals are a low-impact workout... easy on the joints and you can pedal most backwards to give your calves more of a workout too. Most ellipticals also have bars that move with your arms so you get a workout with your arms too.
Treadmills have more of an impact on your bones and feet and joints. Because it's a hard surface, treadmills are more ideal for those who may be training to run a race/marathon. Most treadmills don't come with arm bars that move with your arms.
So I guess for me it is going to come down to quality and price LOL... on the hunt I go!!