I do use a trainer. While spinning would contribute to cardio fitness and perhaps, some to overall cycling ability, using your actual bike on a trainer keeps your riding position in tact and actually exercises the same muscles as you use to ride the bike.

So, if you are looking for fitness only, either works well. If you are looking to ride your bike more efficiently, then the trainer is the way to go IMHO.

You may find it interesting to note that my LBS has leased part of the building to a spinning class. The owner and manager of both stores suggest that there is virtually no crossover between cycling and spinning as far as participation goes. The LBS does see some small benefit in apparel and accessory sales to spinners though.