I have both types sensors (the GSC-10 on my roadie and the accelerometer on my track bike) and a Garmin 500. Here's my reviews FWIW
GSC-10: Much more responsive and syncs with my Garmin much more easily. As Carleton said, the magnet will move when changing gears. Luckily, the magnet setup is pretty straight forward so it shouldn't be a crazy issue to move it around when you change gears but it is an additional thing to do. Also, given that the sensor is zip tied to your bike, it won't be too easy to move should you want to use it on multiple bikes. I would advise getting another GSC-10 if you went that direction.
Accelerometer: Only have 2 rides on it so far so YMMV. So far, I've had issues pairing the sensors with my Garmin (if anyone else has fixed the problem let me know!). I generally can't have bike computers visible on the track so it's hard to see if it's pairing properly. Also, a lot of people have these sensors on their bike at the track so I can't tell what signal I'm picking up! In other words; I need to debug. Otherwise, the setup is dead easy. Simply strap them on and you're ready to go. If you want to use them on multiple bikes swapping them is a piece of cake as well. Performance wise, while accurate, it doesn't seem as responsive as the GSC-10.
As for the bike computer, the Garmin 500 is awesome. Yes, the more fancy ones have bigger colored screens, Strava updates, more data fields per screen, better maps, etc, etc but I've never been left wanting. You can also get power should you ever get a power meter. Best bang for the buck IMO.