I've had a G26 since very shortly after Glock introduced them (serial number BUZxxxUS) and a Shield 9mm for a few years now (about a year after they introduced them and way before the version without the thumb safety). I have many thousands of rounds through the G26 and a couple thousand through the Shield.
As far as build quality, I think they are a toss up. Both display flawless machining without visible tool marks anywhere I can find. I have also checked out a few other examples of both guns and all I have examined the same fit and finish as mine. Both use quite a few stamped metal parts and the Shield uses quite a few MIM parts (the newer Glock do, too, my G26 is old enough that it was way before Glock used MIM). I have detail stripped the Glock many times and the Shield once and can say that all the parts, whether machined, stamped or MIM (or injection molded for the frames), have been of equal quality- well finished, no roughness anywhere I could find, and smooth. I'd say that both are about equal as far as quality of the parts and machining.
As for the finish, the G26 doesn't show much wear, despite being carried and shot constantly for years. Of course, it is old and has the rougher looking Parkerized-type finish with honest-to-God Tenifer treatment on the metal. It does have some finish wear but not much and has never shown any corrosion at all, despite many days where I sweated on it like a pig and didn't wipe it down. The slide and barrel on the Shield have a fantastic finish. I REALLY like it, I have carried the Shield daily for a few years now, either in a rough-side-in leather holster on my body armor when working at my PD job or in a hybrid Alien Gear IWB holster off duty. Absolutely NO holster wear at all on the slide and barrel. The takedown lever shows a little and I have had some minor rust problems with the magazine release inside the frame when I sweat on it and can't immediately wipe it off. The MIM parts apparently aren't Melonite finished. I don't know if they can be because of the way they are made. As long as I remember to take the magazine release out a couple times a year and slather it with CLP to protect it before putting it back, it really isn't a problem.
As for shooting, I shoot both about the same. The trigger in the Shield is a little heavier than the G26 but, for me anyway, the triggers are VERY similar. Both have some light takeup, both have a distinct wall, both have a pretty decent break. Both also display audible and tactile reset, although the G26 is more distinct. After reading about how bad the triggers in the M&Ps were, I was really surprised by how decent the Shield trigger is. Very Glock-like and easy to shoot. Both show about the same accuracy, which is pretty good for how small the guns are. Both have pretty light recoil.
When it comes to actually carrying, this for me is where the Shield shines. The Shield is quite a bit thinner than the G26 and, for me, this makes it easier and more comfortable to carry IWB. With my G26, I sometimes had trouble with minor printing under just a t-shirt when carried at about 4:00 or 4:30 o'clock IWB, where it was the most comfortable. Not bad, just a bit, but it was there because of the extremely squared off profile of the butt and how thick the gun was. With the Shield, the butt is more oval in cross section and, because of the lack of square edges, it conceals much better and, even if it does print a touch if I bend over, the more round profile doesn't look like a square gun, it just looks like a drape or fold in my shirt. I have been "made" a couple times when carrying my G26, I have NEVER been "made" with the Shield.
I never thought that it would make such a big deal, but it is true- the thinner dimension of the Shield makes it MUCH more comfortable to carry IWB. The Glock always felt like a brick on my hip, even when it concealed well. With the Shield, it is enough thinner that I can actually put it on, get it positioned well and forget I have it on. The thinner dimension means that it doesn't bulge the belt and waistband as much and is MUCH more comfortable. I really can't emphasize how much more comfortable the Shield is than the G26, at least for me.
I work and live in a relatively low threat environment, so the lower round count of the Shield doesn't really matter to me. It's only 3 rds, after all, and that doesn't bother me at all. If my family and I are going somewhere that I feel raises the threat level, I'll carry my G19. But, for everyday carry, the Shield can't be beat.
As you can probably tell, I LOVE my Shield. I think it is one of, if not the, best option for CCW on the market right now. Of course, YMMV and you may feel differently. The G26 is also an outstanding gun and would also serve you well. Best option is to see if you can shoot both and see which works best for you. Good luck.
Bub