Re: Glock 34
Gen 4 frame guns are Glock's somewhat recent attempt to keep up with the features offered by the newer Smith and Wesson M&P line. They have interchangable backstraps, larger/easier to reach magazine releases, and a redesigned recoil sping assembly. Others have also noted some changes in small parts like the extractor. Glock has been working out the reliability kinks with the Gen4s ever since they were released.
Gen 3 guns have none of the new whiz-bang features, but are the tried and true 20+ year old Glock design. Plus or minus the tweaks Glock has made since the first Gen1 G17 they are closer to the original.
I would not touch the Gen4 as in my view Glock screwed up a perfectly good design in order to keep up with Smith & Wesson in the marketing war. I'll take the more reliable and somewhat less ergonomic Gen3 every time.
I own a G34 and a G17. The 34s that myself and friends own have been just as reliable as the 17s we own. The 34 is mainly intended for competition, but can be used for duty/defense just fine. It costs a little more $$ and is a littel longe and therefore less concealable, if any of that matters to you.
In my opinion a 17 will serve you just as well if not a little better than the 34. If you have your heart set on a 34, or are planning on doing competitions, then go for it. Otherwise just save yourself some $$ and get a 17.