agreed…
as cool as the 19 and 34 are, the g26 may be glock’s most impressive feat of 9mm engineering. tiny, easy to shoot and 10+1 rounds
This just ain’t right. In fact it’s neglect. Baby Jesus weeps when he thinks of these guns being abused like this. You should send them to my house where they will be shot and/or carried weekly.
What are you talking about? My 19 get’s used. It’s a good first timer for someone who’s never shot a gun before. 
Although I’ll admit to swaying them to an HK P2000 or my USP 9mm.
I’m in the same boat. I bought a 19 about a year ago and fell in love with it. Next I bought a 26. I had some issues with 115 grain ammo with the 26, but it runs 124gr and my defense ammo (Hornady TAP 124gr) perfectly. After I signed up for the January 2011 TigerSwan pistol course, I decided to get a 34 so that I would have an easier shooting gun for the class. I love my 34 so far. Although the 34 has very few rounds down the tube, I have been practicing with draw and point. It has become really comfortable. I will say that sooner or later, I see myself getting a 17. If nothing else, to complete my collection of 9mm Glocks. 
Don’t get me wrong. I love my 34. It’s one of my favorite 9mm’s. Just couldn’t help with the temptation of a new gun.
Tomorrow I’m going down to the gun store to buy some ammo. My P2000 is gonna hit 15k rds this year! That won’t be too hard, she’s at 11k right now.
Find a older Gen2 for cheap. If you’re lucky, $250ish. Take the slide and put it on your 34 frame. Presto!! G17 frankengun. Use the frame from the Gen2 and get a AA .22 kit. Good training.
I will go against the grain here.
I have a G34 and a G19. 3 G19s actually. The 19s seem to be an ideal pistol for many reasons, so I keep spares around.
The 17 is so close in between the two that I would not bother. If I was going to spend money, it would be on a G26 or more ammo.
For me, the 34 just shoots better. I carry the 34 on patrol with a Surefire X200 attached. CCW is a 19. I have no use for a 17, great pistol, just no place for me.
IMHO here’s how it goes.
If you bought (started off) with a Glock 34, or 17. Then get a 26 to complement it.
If you started off with a 19, then you already have all you need.
A 34 or 17 won’t give you any extra capability for a larger package, and the 26 won’t conceal any better.
Get another 19.
“Need” is such an interesting word. I would second everyone saying the only gun you “need” is a Glock 19. If you buy the 17 then you “need” to buy a 26, simply to round out the collection. My suggestion, carry the 34 anytime you find yourself in an open carry situation (training days, range days) and the 19 for all your concealed purposes.