IME, YMMV, yadda yadda yadda
TUNERS
MIM: Cheap, heavy, don't stay in tune very well
MIA: Light, better quality, hold their tune
NECK AND FRETS
MIM: Maple w/ rosewood (?) skunk stripe, rough frets, thick gloss finish
MIA: Maple, graphite reinforced, rolled edges, smooth frets, thin gloss finish
BRIDGE
MIM: Bent metal, cheap saddles, bridge-load only
MIA: High-mass, solid saddles, bridge-load or thru-body
PICKUPS AND ELECTRONICS
MIM: Cheap, some noise, decent pots
MIA: Not as cheap, less noise, decent pots (I'm not a fan of Fender's electronics but have yet to change them)
WOOD AND FINISH
MIM: Alder or ash (limited), thick finish
MIA: Light alder or ash (not as limited), thin finish
PRICE
MIM: Ripoff new, good price used
MIA: Slight ripoff new, great price used
OVERALL QUALITY
MIM: Decent, somewhat difficult to find a good playing one out of the lot
MIA: Good, easier to find a good playing one out of the lot, but bad ones do exist
Personally I'd rather buy a Lakland, Lull, Sadowsky Metro, or build a parts bass if I had to get another J.