I own many Mozart opera and chamber music recordings, but am less interested in the symphonies and unlikely to get a complete set thereof.
That said, FWIW (which may not be much), I'd go for the Mackerras. Based on the recordings I've heard and a couple of Mackerras-conducted operas I attended, he's a fantastic Mozartian (and has a reputation as such). I doubt you could go badly wrong with him.
[Added] I own a double CD of Bohm conducting late symphonies (35-41), and it's good, but I'll look for other interpretations if I add more recordings.