15% is more effective and a stronger concentration of minoxidil.
15% contains 3 times as much minoxidil as 5%. This does not mean it is 3 times as effective.
5% minoxidil was shown to be 45% more effective than 2% minoxidil. 15% minoxidil will be more effective than 5%, but it won't be that much.
Only use 5% for when you start out, then once you see it is not working and you are not getting side effect, consider switching to 15%.
Just my opinion, I am no expert.