As in any purchase, talk to your friends and neighbors -- who do they use? Are they happy with them? Check with the Better Business Bureau for complaints. The BBB can't prevent shady business, but they can and do keep a file of complaints filed by people who have had dealings with them. Remember, just because the unit or Company carries a brand name is not any indication that the unit is any better.........but it may mean it is more costly for you!
Ask at least two to come to your home to look at your plumbing and then give you a quote on their equipment. Have them explain all the features of the unit, as well as the warranty.