Sustain is a little better in a 35" , but the action is a little sloppy - if you play fast or funk or Jaco-like stuff you will need a 34" . If you play 80's rock ballads like Journey or Foreigner you'll love a 35".
the 35" strings will vibrate a little wider and if your bass is not set up properly, strings tend to buzz against the frets if you pull too hard, especially near the mid section of the bass around the 12th fret or sometimes the near the nut were the strings a just a few mm away from the frets. - in my experience, when you own a 35" you get a better fuller natural sound from the added length of string and more vibration. but it can be annoying playing hard and fast simultaneously. slapping takes extra precision on a 35" if you play fast.
34" is widely accepted as the best ballance between good action and a playable tight sound and enough string resonance. bigger than 35 and your strings turn to noodles. shorter than 34" and you loose sustain and warmth.