Since you don't say for what purpose you will be using your Glock, it is kind of hard to give a firm answer, but I'll try.
I own, carry, compete with, shoot and like both the G17 and the G19. Ultimately, you can't go wrong with either one.
Their dimensions:
Glock 17:
Length: 7.32"
Height: 5.43"
Width: 1.18"
Weight: 24.79 oz.
Glock 19:
Length: 6.85"
Height:5.00"
Width: 1.18"
Weight: 23.45 oz.
Given that the G17 is the same width as the G19 and is only 0.43" taller, 0.47" longer and weighs just a paltry 1.34 ounces more than the G19 while providing two more rounds of 9mm ammunition, a longer grip surface and a sight radius that is 0.47" longer than the G19, I prefer the G17 over the G19 and carry it the vast majority of the time because I find the differences in their dimensions to be insignificant to me when I carry.
The longer grip of the G17 just feels more comfortable in my hands and having the 0.47" longer barrel on the G17 to squeeze out that last little bit of velocity is just icing on the cake so I carry the G17.
If you can get away with the concealing the little bit of extra length in both dimensions (if this gun will be used for that purpose) I would encourage you to go for the G17. On the other hand, you might be constrained to the G19 due to those factors.
If, however, it is to be used simply for recreational purposes, then either will suffice of course, although the G17 does offer the advantage of an increased sight radius which helps (at least in theory
) to increase one's accuracy.
Your call.