Voted for b3 and for a legit reason... It gives me one heck of a challenge and to me b4 feels stale compared to its other version.
The grievances from the fireballs are nothing to me anyways. Just go heavy... If you run into a fireball then you only lose half your hp and player collision is much more difficult on you.
Demomen can also caber the gap.
(Pro tip- Did you know a whip boosted heavy with a G.R.U. can jump two whole coins?!... Time it right and you avoid the coin trap and both fireballs all in one jump!!!)