The 150 is internal focusing, the 105 isn't.

It's very useful to have a tripod collar on a macro lens.

but I don't want to go overboard. Just for fun right now.

I recently did some research and bought a canon f2.8 100mm USM macro. It is not a budget model but I'm using it professionally with the MR-14 ring flash. I learned that the 60mm costs less and takes excellent images as well. The difference is that the 60mm needs to be closer to what you're photographing. This may or may not be important depending if it is going to block your light or make the subject uncomfortable.

Tokina makes a less expensive lens that got a positive review in Popular Photography's Dec issue (i think that was the issue), I'm not sure how the AF is with Canon.