GPS watch is fine for tracking where you have been. The GPS is probably more accurate than the GPS in your smartphone. Some of the GPS watches have a built-in altimeter, which is a great feature. The main advantage of a dedicated bicycle computer is the bigger screen is much easier to see when you are riding. You can program turn-by-turn directions into many bike computers, while the watch is again too small to do this effectively even if it is available. For casual training, you may not need to see the screen when you are riding; just download the data after the ride. It's up to you.