I couldn't easily make wine without either. It is not a matter of which is better. Rather, it is a matter of which one is better in which circumstance.
As was already mentioned, StarSan is a contact sanitizer, k-meta sanitizes via fumes. Use each accordingly.
Examples:
Sanitizing a carboy - Add k-meta, stopper the carboy and shake it around. The fumes will do a great job of sanitizing.
Stir spoon - Spray it with StarSan, give it 30 seconds contact on the spoon's surface and it is ready to go.
In theory one is supposed to let the StarSan dry on the surface. I never do. After I apply the StarSan and let it set, I also rinse it off before using... that's just me, though.