Most musicians agree on the notation of a minor chord. Just an m will do, so we will write
GM7. Unfortunately, some musicians use min instead. And in some books, like the New Real Book and the Latin Real Book (both published by Sher Music), mi is used.
So how are these preferences devided? Just for the fun it it, I did a small investigation in Google. I searched for the term Am7 and I compared the number of hits to Ami7 and Amin7. And then I did the same for Bm7, Cm7, etcetera. Here are the results. The total number of hits on m7-chords is 4.941.000, on mi7-chords 201.400 and on min7-chords 120.660. The numbers are clear. The notation method with m is far more common than the other two. As for the two alternatives, min is used a little less than mi.
We may conclude that m is the abbreviation musicians have settled on. Therefore, publishers and musicians, if you want to comform yourself to the standard, use m!