Mier products, Dakota alert and others are an easy integration into the M1 wirelessly, not to mention they are far more asthetically pleasing. HAI's sensor is a universal unit with their own transmitter installed. I forgot who makes it, but there's a "universal" version out there that can have a system transmitter installed.
I'm a dealer for both products and I've had far better experiences talking to Elk and the engineers over there working on integration compared to HAI. I've found that HAI is nice if your end client wants to maintain a straight across the board platform and integrate into some manufacturers, but wants a single name on most of their hardware. When there's an issue, it's been easier to get an answer from NC over LA. I've installed plenty of each in multi-million dollar projects over the past few years that the M1 has been out, and by this, I mean jobs where the security system budget alone was 6 digits, not including integration and automation.
The one place that HAI is somewhat stronger IMHO is integrated audio and some of the out of the box simple lighting setup.
I've found that Elk is more robust in the security end while HAI has a couple of stronger points in the HA side. I explain it to clients as there's Chevy and Ford. I view Elk as a security panel first that does automation and other functions, and HAI as an automation panel that does security, and you have to view which is more important to your application. I've found that Elk has better troubleshooting tools that are of value for a pro dealer/installer, as well as their smart power supply, albeit it's limited in power output, it doesn't need to eat zones for supervision. Pros and cons for each platform, like I said before. I think IMHO Elk has their engineering together for the pro dealer and integrator, especially when you look at the troubleshooting and testing sections of their panels.
The big thing is cost, you can get a pretty nice Elk setup for less than the HAI panel, but if you compare apples to apples with hardware needed to level the playing field, then you're looking at the same cost overall.
The big perk to Elk in my area of the country, with retrofits and integration NOT being new construction is the fact I can wire modules (99% of the time) anywhere on the 485 bus and not have to worry about connecting it all at the main board, which HAI is a must. We're limited at times to having a piece of a closet on each floor or small utility space somewhere, in addition to the normal electrical spot, so in those cases it is far easier to integrate an Elk with other hardware when space is a premium or connecting to other vendors to provide automation on one platform.