We have to explain that there are differences among refinishers, and that it's a great product if it's done with the right materials and techniques. Liners have their drawbacks, too, particularly with regard to esthetics. Some people like the clean, simple lines of the liner look, but if you've got an older tub with some character and definition in the skirt, liners have a hard time replicating them. In addition, liners are plastic and they look plastic. And if there is leakage around the liner and water gets under it, it can be squishy and smelly, and difficult to deal with.
Warranties vary widely for both products.
Also, read the warranty on your contract. What they are covering is just as important as the terms of the warranty. An unconditional warranty is a lot better than a limited one. Both industries shy away from working on fiberglass tubs, and most liner companies won't do jetted tubs, nor will they line claw-foot tubs. Chipped, dented or even rusted-through tubs are no problem, however, but be prepared to pay more if repairs are necessary.