Decimate completely removes any UV data. If you want to go reduce your tri count, delete edgeloops, remove faces that can't be seen, remove details, etc. Most of the stuff you'll do will need to be done manually.

even if decimate worked with uvs (some paid software does it) i highly recommend doing this manually anyway.
the idea is to get the best possible representative geometry of your hi/mid model with the least amount of tris, which really can't be done by any algorithm (..yet?)

it's more of an art than a calculation; as helljumper recommends, get rid of anything you don't need, anything you won't see, and simplify anything that's not particularly obvious or which can't be approximated with normal or other masks.

also, keep in mind most people will only ever see your model from the in-game overhead view, so it's only critically important for your model to look really good from that particular angle.