I would say it's more useful to take teleport if you're solo top because:
(a) you're solo-ing the lane, so nobody can cover when you leave.
(b) It's a much longer walk than mid: if you need to desperately go b, you need teleport so that your opponent doesn't push down your turret.
Teleport isn't as useful mid, imo. If you're playing an AP caster you need ignite to finish off that 40 health they got away with, because your burst is never enough to 100-0 someone that early on: only if you're playing LeBlanc and your opponent is bad.