Take your time, as it is a big investment (both in money and time) and there are a lot of choices out there.
I recommend joining the homequiltingsystems group on Yahoo! and reading both their "files" section and back posts. Many people on that group have admitted to being overwhelmed after buying their first longarm. The general advice is to take a full year researching systems before making your first purchase. You should actually try out different systems at quilt shows, for one thing. Also it is often recommended to buy from someone local so you can get help setting up and getting started; having a shop where you can take classes is a big plus too.