Well, I don't think you'd go wrong with either the M&P or the Steyr. If you are a big "low bore axis" fan, then the Steyr is going to be your choice. The way the grip is designed it leaves the much touted Glock looking like a Sig P226 in the bore axis department. The grip on the M&P is adjustable with its various size backstraps but the grip of the Steyr is (at least for me) pretty decent as it is, even if it's not adjustable. What is going to be a big difference is that the M&P has more of a 1911 grip angle while the Steyr, like the Glock is more Lugerish in its feel. If you're not used to the difference in the angle, you will feel that the pistol points higher and will slow you down a little until you get used to it. However, once you are, the recoil I think is a little more manageable.
Moving on to the slide, one of the big differences is going to be the sights. The M&P uses a traditional 3-dot variety while the Steyr uses their own unique design. It's one of those love it or hate it affairs but I think once you start getting used to them, I found myself moving more into the love it camp. Reminds me of an ACOG scope reticle in a way. LOL However, if you don't like them, you can always swap them out with a conventional tritium version. As for the slide itself, the slide of the M&P is easier to grasp. due to the lower bore axis of the Steyr, it means the slide is a lot lower as well. I'd put it on par with the CZ-75 I think would be a close comparison. It's the price the design pays for the barrel sitting so low in the pistol.
Moving on to the frame, well, the Steyr with its bull nose isn't going to win many beauty pageants! LOL Another thing that I'm not thrilled about is the key lock. It's totally worthless to me but you need to depress the lock button in order to rotate the takedown lever. The M&P doesn't have this issue but really, it's not that difficult to deal with and how often are you going to be concerned with speed disassembly. The slide release is ambidextrous on the M&P but righty friendly only on the Steyr. Might be an issue or maybe not. I do think that the trigger on the Steyr is better than the M&P, especially the factory M&P trigger. An Apex trigger helps but also adds to the price tag. As for magazines, M&P mags are always going to be easier to come across but the Steyr magazines are excellent quality.
Now as far as function goes, I've never had a problem with the M&P. The Steyr had a few failures to eject with the first 200 rounds. I attribute this to a break in period though as the problem got less common as firing progressed and now it's not an issue. Just for good measure, I did add a cut down 1911 safety plunger spring inside the factory Steyr extractor for a little added tension and enhancing the grip the extractor exerts on the cartridge rim. But it's like I said, with the Steyr, there may be a break in period that I never had with the M&P.
Finally is the aftermarket support. Here is where the M&P blows the Steyr out of the water! While the M&P hasn't got to Glock proportions in this area(yet), the Steyr is a LOT more limited. Even holster selection for the Steyr is a bit more sketchy but I have gone with Blade Tech kydex holsters and have been quite happy with them. But if you are looking for something like threaded barrels and the like, the M&P is the natural choice here.
Really, I don't think there's a wrong answer for either one and it all comes down to what you prefer and what fits you better.
