I eventually plan to add a full size compact 9mm. I think the Glock 19 would be one to try, just because, or just to see what all the interest is in this particular model. But all of the Glocks seemed to be priced in the $500-550 area. Whereas, the 2.0 M&P Compact is already in the $450 range and at $399 on at least one website.
I'm sure the Glock 26 and 43 models are great to own and shoot. But the M&P Shield can be found everywhere in the $350 range and often below $300. No wonder it sells.
Your references to size confuse me as to what you want. "Full size" and "compact" are two different things. (I like the term "mid-size" rather than "compact," but "compact" is more common, so I'll go with that.) Meanwhile, the Glock 26 and 43 are both subcompacts (double- and single-stack, respectively), so you've really referenced three sizes, almost the full range of handguns.
I have a Glock 19 Gen 4 for a couple reasons. First, because it's the standard-bearer in its class; second, because at the time I bought the Glock, the S&W M&P9 compact was the wrong size, smaller than a typical compact, but larger than a subcompact. I did not like the original M&P compact at all.
But the new M2.0 compact is the perfect mid-size (essentially identical to the Glock 19), it has an improved trigger, and lotsa people think it's ergonomically superior to the Glock. (Plus, as you've noted, the M&P costs less than the Glock.) The new S&W compact is a huge improvement in my mind.
So now I'd like to add an M&P9 compact to my collection.
However ... if I were starting from nothing and wanted only one mid-size ... it'd be the S&W M2.0.