The tall ones are the old ones; I have one from 2007 and they’re a bit old hat now to be honest.
Then they shrunk vertically but kept the same processors, core2duos; you can stick more RAM in them (mine is limited to 3GB which is rather annoying) but they’re not that different.
The newer ones are the tiny ones without a built in CD drive; much faster processors (i5s and i7s), more RAM, the power supply is built in rather than needing an external brick and they are just super shiny and small. I want one, but the budget won’t stretch far enough…
I’m amazed the old ones are still so expensive, mind; they’re only a tiny bit cheaper than the new ones!