Oranu Warriors make the best Mech Infantry, Impi make pretty awsome Mech infantry too. If its all about the short game I can see Preats, but for troops that are truly worht building a hundred of to make deadly long term armys they give no advantage. Berzerks make a good upgrade too as already mentioned, esp as you can take the Mech infantry path instead of the Marine path for an even stronger unit.
If you can get the tech advantage Drill II-IV Infantry lose very few HP vs less advanced troops.
If you can get the tech advantage Drill II-IV Infantry lose very few HP vs less advanced troops.