I spent the April 16th weekend in Georgia at the USPSA Area 6 match, and more importantly, with Dave Sevigny and Randi Rogers. Both shot the Glock 34 in Production class. They like the longer sight radius the 34 offers. The longer sight radius is preferred in competition because of the need for accuracy to achieve higher scores.
That's the obvious advantage as you pointed out. There should also be a difference in the feel during recoil, though I have not shot them side by side for comparison. The 34 and the 17 have slides that weigh the same (11.7oz), but the recoil impulse can change dramatically. But you might not be able to see this without the aid of a high-speed camera, so you'll have to go by feel.
The other benefit is one of confidence. If you feel more confident shooting the 34, and feel you can me more accurate with it, then that's the one you want to go with. Never underestimate the importance of a solid mental game in competition.
For reference, go to page 19 of the IDPA rule book
and you'll find the rules governing firearms in the SSP division.
Hope that helps. - ERHARDT
BTW, regarding the 17, take a look at the SIRT Training Pistol
. It was developed my Mike Hughes. I got to play with it at SHOT Show and if I were shooting a Glock in competition I would definitely pick one of these up to augment my training.