Agnosticism indeed doesn't imply believing nonsense. But being agnostic about a question which can be easily decided on evidential grounds has similar consequences as believing nonsense, compartmentalisation-wise. For example, it requires believing that it would take five days to arrive to a reasonable conclusion, which belief may be false. On the other hand, for the particular question asked by the OP the five days may not be unreasonably long, so you may have a point.

which do you believe is a better model of the universe, M-theory or loop quantum gravity?

Loop quantum gravity. (I don't imply that this is a question one should have a definite opinion about.)