I have a GPS (GolfLogix) and I like it a lot. I think it's the better choice if you can only get one. The reason I say that is because I play with a guy who has a laser and depending on the course, his laser is not useful on quite a few holes. We played at a course a couple weeks ago that had a lot of doglegs, elevated greens and blind shots. He was simply out of luck on some of those holes while I had no problem getting yardages from my GPS.
Now if I had the money I would also get a laser to use at the driving range so I could see exactly how far the targets are from me.