Dedicated bike mounted units enjoyed proven accuracy for years that cell phone based apps are still struggling to achieve. Not to mention the advantage of seeing the data real time as you ride on a compact, function specific easy to manage unit.

This approach also allows you to keep the more expensive, and fragile device tucked away until/unless it's needed.

BTW- you probably already own the only performance measurement device that matters, a wrist watch. If you're doing the same distance faster, then you're improving. If not, then you're not.