I thought the the M9 has a worse handling of third party batteries.
Really? For some reason I assumed it was better than the terrible situation with the M8, but it makes sense that nothing wouldn't have changed.
If I did it again now, I'd go with the M9, with a known good sensor. M8 had the shutter and screen issues (mine had both), m9 had the sensor rot.
But the M9 has a hair more sensitivity, a few more megapixels, but you keep your wides, which can be a plus or not. And most importantly, no hot mirror filter needed. They're hard to find, pricey, and a pain to keep switching from lens to lens.
A while back I shot a wedding with the M8 and couldn't find a 39-43 step up ring anywhere (mine came with two 43mm filters, guess the previous owner was a Zeiss guy). Sensitivity and file size weren't important; it was outdoors and the couple didn't want big prints.
But man, the groomsmen were all wearing rented polyester tuxedos, and every single one looked purple. That was a LOT of editing.
I went with the M8 since, at the time, they were going at times below $1k, but the M9 was still probably around $4k+ not long after the M240 came out. Nowadays it looks like the M9 has come down a lot, but the M8.2 has come up a bit (seeing one on eBay right now for $3600.