I've owned both a 17 and 34 (still have the 34), and have shot a 17L. The differences I've seen have more to do with how they feel in your hand, balance, etc.
The longer 34 slide and barrel didn't make for a more accurate gun, but the longer sight radius might make it a hair easier to shoot well. (The 34 is balanced a little better; essentially the same weight as the 17, but more balanced between front and back.)
Some very short-barreled guns can be very accurate; barrel length isn't a big factor in that aspect of performane.