From fairly poor judgement, since I have ridden one and not the other, I shall say Manta.
I have ridden Tastu, it was okay. The pretzel loop was awesome, but the rest of the ride was dissapointing I do have to say. It was then that I realised that 'flying' coasters are for the experience, not for the thrill. Since Manta looks like it delivers quite an experience, with theme and everything, and also is 'designed to flow like a Manta', that wins the vote. Quite a coincidence that Manta's swim like Pretzel loops.
What an odd choice for a pair of parks. SFA, hands down. LaRonde has the worst operations of any 6F park I've ever been to with MM being the second worst. I sincerely doubt that 6FMex can be worse, and it's the only one I'm missing.