What I usually hear is that you want a 10x turnover rate. So around 150gph for a 15g. Although you can almost never have too much filtration as long as the flow isn't too strong for fish to swim, it will depend on what kind of fish you have, if your tank is planted or not, and how heavily you are stocked.
The aquaclear 50 is 200gph. It's also a good idea to have higher filtration because if you ever decide to upgrade to a larger tank, you won't need to buy a new filter.
I have a 125gph filter on my 10g tank, and a 45gph filter on my 4g tank.