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.
Bling Vs. Samauri