Use the 20 and use the proper 15 amp breakers in the panel. The 20 refers to the rating of the GFCI-- which does not provide overcurrent protection, only ground fault protection. Using the 20 amp faceless GFCI complies with code in your case.
If the GFCI did have the receptacle faces, it would be a code violation to use 20 amp receptacles on a 15 amp circuit. But in my opinion, doing so does not create any safety issue.

"If the GFCI did have the receptacle faces, it would be a code violation to use 20 amp receptacles on a 15 amp circuit."

It would be a code violation to install receptacles regardless of the rating. Jetted tubs must be dedicated. If a receptacle is used, something else can be plugged in. That's the entire purpose of a faceless.

Awesome! Thanks! I'll install the 15A breakers and use the 20A faceless GFCI's.