I'd probably opt for a sunny, but cool, plant room over a dark garage. What are the temps going to be in both locations?

I would definitely go for the plant room. I store mine in a room with lots of sun, but the temps in there are on the cool side (low to mid 60s) and they really like it in there because they get some sun, but they also dont get too much heat which would cause them to start growing again. A dark garage would probably cause them to rot. I only store bulbs in my garage and the most cold tolerant plants because everything else doesnt seem to tolerate it in there.
Good luck!