Compared to other recent 3D conversions, Thor is exceptional… Great summary of the European release reactions here:
Compared to the 2D “native” version of Thor, it is an inferior experience, at lower resolution, and with muddier colors and less dark blacks.
If you can only afford to see it once in the theater, I would recommend the 2D version. Your brain and eyes will have much less visual decoding and recompositing work to do, and thus will be more fully free to absorb the storyline and “splosions“.