As I said before, they're pretty much similar, where and how much you pay for additional conveniences is usually the factor. There's no silver bullet and where one is stronger, the other has another area where it's stronger.
It's been a little while since I've used an OPII, but from memory, all the serials are limited to the board. I'd much rather work on an M1 with glides installed in it's own enclosure than an OP where all the boards and inputs are "stacked". Enclosures for expansion aren't a deal breaker, and in many of my installs, I've ordered the M1 kit with the 14" can and then a second 28" for the main board, using the smaller for subpanels. IMHO, there's a limit to how big the can should really be anyways, I'd rather stack a pair of 28's or mount side by side before moving to a 42" personally.
I relate HAI to "Apple" and Elk to "Android". If you want to pick a bunch of single manufacturer boxes and get a system running with no thought or integration, then it's a HAI install. If you need to integrate more 3rd party products and can relate to how X, Y and Z inter-relate and communicate...Elk.