1) MULEs mine three times more minerals than SCVs.
Source quote: "Currently the MULE mines three times the minerals of a normal SCV per round."
Source link: [
http://sc2pod.com/news/Fan_Site_QA_3__SC2Pod/ SC2Pod Q&A].
I seriously doubt MULEs will continue to mine at that rate.
2) Get info on the planetary fortress
here. Again, stats are always in flux, but beefing it up might not work. If you buy one, it's wasted if the enemy does not attack, while you could use the extra resources provided by MULEs to protect new expansions anyway and probably still have enough to get an edge in other areas (eg offense, scouting, whatever).