Which barber you use is almost entirely dependent on how consistently and how long you can keep minions in vulnerable.
The basic idea is, you need barber to pop before vulnerable expires, otherwise you lose a tone of damage.
If you play a class that has crappy options to make minions vulnerable, like sorceress, then you are likely reliant on the exploit glyph to make minions vulnerable, and that glyph lasts 3 seconds, so that means you need a sub 3s barber.
Otherwise, if you can keep minions and bosses constantly vulnerable with ease, say like druid, you can go for the full 4s barber and enjoy the extra damage, albeit additional latency.
Other than vulnerable, the only other consideration is kill speed for speed farming. Once you account for both those factors take the longest barber you can get.