First off, Grindwater doesn't belong in this conversation......high end, I think not. I can assume, from your board selections, that you are looking for a surf style board rather than skim? Maybe give us some more details about your riding style and boat/wave setup. What are you riding now?
I know a couple guys around here are killing it and really liking the new Elevation board from Shred Stixx. I haven't ridden one, and therefore can't comment. I, also, haven't had the opportunity to ride the new Flyboy, however, I do have the TWP James Walker board and wouldn't trade it for anything BUT the new Flyboy. It's my understanding and observation that James and the Flyboy crew are attempting to take wakesurf boards to the next level, and I feel like these guys are going to start paving the way for future improvements in shape/design and materials for board builds.
You really can't go wrong with either of the (2) boards you mentioned, but, if I were buying a board right now, it'd be the Flyboy. Heck, if I didn't know for a fact my wife would have my jewels in a jar on the counter, I'd already have the Flyboy in the rack! Just for s's and giggles, here's a picture of my TWP James Walker Sig.