Personally, the 34. The 34 effectively replaced the 17L, and I like the newer generation glocks. The 17Ls are typically harder to find since they do not make them anymore.
Glock 17 Gen 4 because it's better balanced. I was set on buying a 34 but after holding both the 17 felt better.
If your planning on shooting IDPA with it, the 17L is not legal. If not than it's personal opinion. The 17L is kind of cool since it's more rare.
. old old thread, but..... I was originally looking for a 17L (because of the 9mm "thing"), couldn't find one so I bought my Wife a 34 then, just 'cause, I bought a 24 for myself and haven't looked back - like mucho the .40cal round