Personally I weighed the two guns; and I prefer the P226 over the G19.
Preformance: Pretty similar, admittedly it's very easy to get a FMU slide for a G19 vs. a FMU slide for a P226 for propane useage; but I find that the newer TM GBB's work admirably under propane and I get a nice clean 'crisp' recoil from both. Pretty much same recoil, and the same rof.
Ammo: 25 rounds vs. 20 Rounds, P226 sort of wins on this issue, sort of.. (See below)
Magazines: Magazines are far easier to obtain for the KSC though, so keep that in mind, but the TM mag design really does shine through. The bottom hole on the P226 mag makes mag loading a cinch, wheras the KSC features a locking mag follower, which helps in ensuring double-feeds, but I sometimes find this troublesome (esp. when you try and pre-load a round, it sometimes causes the mag follower to stay in the locked position).
Not to mention: TM's flexible polymer/plastic feeding head basically prevents the nasty problem of accidentally spraying bb's all over the place which I have done on more than one occasion with the Glock and other GBB's. But: This makes them sort of a hassle to unload them for storage..
Hop Up: KSC's dial-o-matic vs. TM's under-barrel dial. It's sort of a tradeoff; one you really need a tool to adjust but can be done with the slide still on, vs. TM's where it's just a simple dial on the bottom of the outer barrel, but needs the entire slide assembly taken off to adjust.
Accuracy: I found them deadly accurate on every occasion that i've used both guns. Admittedly; I was using a severely battered/beaten up KWA G19, but it was remarkably accurate in the CQB zone where I was in. I even used it against visiting PPCLI infantrymen who took all the rental armalites, and left me with the rather amusing G19, and it worked great.
P226 I also find is a straight shooter through and through. Still haven't had a problem with it yet (whereas I've had problems with damn near pretty much every single other gun that i've owned), but it's a pretty dead-on shooter.
Mag Cooldown: I didn't really have issues with mag cool down with either gun (there were a few times, but that was chocked up to under-filling),
Features: TM's 226 does feature a very nice decocker, well within reach of your thumb (provided you fire the gun with your right hand) and fairly easy to use safeties. Glock is.. Spartan, but that is one of the major attractive features of it.
However, some people do enjoy the exposed hammer of the P226 vs. the closed-hammer of the glock, mainly because you don't have to cock the gun to just cock the hammer.
Upgradability: There are plenty of new frames/slides and upgrades for the P226, but there are pretty much the same number for the Glock (admittedly less metal parts if you like that type of thing), but otherwise nothing really substantial stands out between the two. (But the P226 has a threaded barrel that WORKS!)
External accessories are diverse, as they both have a rail (admittedly the P226 has several grooves vs. the glock's single groove) for mounting things like tac-lights.
Size/Shape: Between the two, the G19 is a wee bit smaller (in my opinion), but I haven't held them up side-by-side yet so that's sort of up for grabs. And of course; if you like boxes, go with boxes, if you like.. boxy curved items.. go with.. You get the picture
Sight Picture: It's sort of a tie, I suppose it depends on what picture you want. Glock has a nice neat U shaped groove, painted white on their rear sight with a single dot on the front blade. Rear IIRC is adjustable (and has led to many a lost rear sight pictures), and both are replacable with fibre-optics (which are expensive)
Vs. The P226, wherein it has permanently fixed three dot sights (meaning two dots on the rear, one dot on the front blade), makes also an attractive sight picture (admittedly, there are circular grooves on the top of the sights themselves, which I think are white on the real steel version, but remained unpainted for the airsoft one) and IIRC are also replacable with real-steel tritium sights.
Grips: P226 has a nice pebbly feel with two solid grips on either side. Glock 19 is just a simple polymer frame with light finger grooves; but both equally go well with hogue handall grips (or if you're more of a wraparound type, apperantly the hogue full grip will fit on the P226 with sufficient modification), I find the glock grip too is sort of boxy, but it'll work either way.
Personally I side now with the P226; Not only is it a kickass weapon used by people like Jack Bauer and Neil McCauley, but with 5 more rounds than the G19, and it's very unique styling; i'd side with the newer gun. Plus; you get decent trades to boot!
Edit: I posted way too much stuff: Go with the P226!