I have a trainer (had one and used it heavily for years), but I must say I much prefer spinning classes.
If the place you go has only one class at a bad time, maybe consider a different place with better hours.
The instructor, bikes being used, room setup all are very important, any of those things is bad and the classes can suck.
I find with spinning, because the classes are scheduled, I get more motivated to attend as opposed to hitting the trainer.
In the end anything that you keep up with that gives you bike type cardio good. Key being sticking with it religiously. I find with fitness you need to keep at it very regularly, doesn't take much to fall off the wagon.