There is very little difference really.
They are both compressed lossless formats. So they are both identical to original source material. The compression levels/efficiency of the two are VERY close if not identical so one is not offering a better compression rate really.

I believe the DTS-MA has an additional track that is compatible with older receivers and TrueHD supposedly has a greater range of metadata available. Other than that, there is no real reason to go with one over the other for the consumer. You're saving the same amount of space and getting the same amount of quality and the same sound.

It all boils down to which name do you prefer saying when you have to explain it to the non-audophiles in the family. ;)