>>95564232
MHE
Moved away from in-universe time measurement to the "n times per scene" type
Adds mutant PCs, but as a very obvious "you like Shadowrun, right" ploy. Also adds secondary classes
Has a bunch of rules for things the other editions are missing, like underwater operations and playing trader
Has an advanced skill challenge system lifted from Gun Dog Zero (IIRC)
Setting advanced again, separating the Hunters (PCs) from a single block of general-purpose mercs to the small-settlement-aligned Good Hunters (the PCS) vs the megacorp-aligned Bad Hunters. Not a fan of this development, personally.

MHd20
Don't bother, it sucks. Look at the cover art and maybe the opening manga, but I can't think of anything worth stealing mechanically. It's not even fully compatible with other d20 system books.

Personal advice is to use Maximum as a base, then steal shit from Extreme and MH1 to taste.