Playboat. Low gradient big water has the same form even as flows get higher. If you were a tenative class III boater, maybe a bigger boat would help, but for most boaters, a playboat is fine, and a hell of a lot of fun. There will be some great surf in there. The whitewater of the grand is hyped up to mythical proportions (is class 10!)... but the reality is that its mostly large volume class III. For a kayaker with a good brace a good roll the grand is not that hard.
Truth is that playboats are easier to get through big wave holes than bigger boats once you learn the technique (hit the huge breaking wave holes with a 10-11 or 1-2 o'clock angle, downstream edge, active paddle blade planted in the downstream foam pile to pull you through, while head butting the wave). Standard T up to the wave and punch results in back enders almost every time.
When in doubt... take the meat line!
Flatwater is not a big deal in a comfy boat like the superstar. The scenery will keep you entertained.
Also, getting throttled in a playboat in the large rapids is really part of the fun on big water. Its low consequence high fun factor, so don't worry too much.