The only difference between a 20a and a 15a is that you CANNOT plug a 20a
device into a 15a outlet, so you can't overload the 15a circuit. (well, at
least not that way)
Since you probably don't have any 20a devices, it makes no difference what
you use; as long as you don't have 20a outlets on a 15a circuit.
The only exception is that if you have only one outlet on a 20a circuit, it
has to be a 20a outlet. I haven't any idea why.
It goes without saying that you still have to use #12 wire with 20a breaker,
regardless of the outlets.