Just FYI...for kayaks, shorter means slower. If you're planning to battle current, a longer (and thus faster) kayak may be a better choice.

More important though is to try before you buy. Lots of places rent 'yaks for the hour/day/weekend/etc. Try a few models and spend at least 30 minutes in the seat out on the water. You won't find the places it starts to get uncomfortable unless you spend some real seat-time in each model you're considering.

Good luck.