I imagine that this answer is probably irellavent at this point since it was months ago, but since no one else seemed to give an opinion I guess I will.

I would say go for the trainer. A GPS is not at all necessary, but using a trainer might help you get more time in the saddle if you are time crunched. You can always use something like mapmytri.com to figure out how far you went and if you have a cheap watch you can figure out how fast you are running. I would even say an HR monitor would be more important than a GPS.

Sorry this is so late coming, but I HOPE that it helps.