Each project seems to differ a little in their aim.
From what I seem to remember from reading... (have to do some reading up to get up-to-date)
(I think all these projects were demonstrated to be real)
Minimig wanted to just emulate the A500 in how it functions and in timing (want to be as fast, not faster than the original). It's an opensource project and there are allready people building them.
A-clone wants to emulate the seperate (custom) chips so they can make drop-in replacements for each chip (100% same working and timing). Also it's a commercial project.
Then there's one which focusses on having an a fast as possible Amiga clone with AGA support.