The shape of the grip is ever-so-slightly different between the two models. The arched "hump" at the bottom-rear of the grip is moved upward as the grip is shortened on the various models of Glocks, so the arch is closer to the top of the grip on the G19 than the G17, and on the G26 it's higher than the G19. It changes the "feel" of the weapon in a person's hand just enough that some folks will really like one model and not like the other one(s) at all. It's not just the size of the grip, or size of the person's hand, either; I know several of 6'4" or taller folks who prefer the G19, and some five-foot-nothing shooters that really enjoy their G17 pistols. The only other obvious difference is the number of rounds in the magazine. The G17 holds 17, and slightly shorter G19 has a 15-shot magazine. There are minor interior differences (the barrel camming surfaces and locking blocks are different, for instance), but many/most of the internal parts will interchange between these two models.
Originally Posted by cruzthepug
If you want something a little bit different, both for shooting and just owning, have you ever considered getting one of the factory-ported Glocks? The "C" models (the C is appended to the end of the normal model number, such as in G17C) have a distinct recoil sensation; compared to the regular models of the same caliber and size, the ported guns bounce/flip much less during firing. In the 9mm calibers, it's almost like shooting a blank-firing gun. You hear a loud noise, the slide cycles with minimal muzzle flip, and you're back on target, ready for the next shot. With cheap target-shooting ammo, you often get a flash from the ports that is visible from the side and rear (although the shooter rarely notices it), and with all ammo, you'll hear a louder-and-sharper-than-normal "Crack!" when the pistol is fired.
I prefer my G19C for general range shooting and fast strings of accurately fired shots. Here is a photo of one of my co-workers shooting it at the local indoor range:
The "C" models are not offered in the Gen4 Glocks, so if you want a factory-ported Glock, you'll have to get a Gen3 model (or one of the earlier Gen2, or fairly rare Gen1 ported model 17L competition pistols). Also, the ported models are generally offered in the Compact (G19-size) and full-size (G17) models; no subcompacts have ever been factory-ported. A few of the long-slide competition models have been factory-ported (some are labeled as "C" models, some are not), such as the Gen1 17L, and the Gen2 and Gen3 models 24/24C in .40 caliber.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)