Darui hits twice as well with his field up.

I prefer mifune's mystery ability - eating buffs is a great feature. Darui's paralyze is nice, and hits quite a few ninjas, but normally when you run a lightning team you have other methods of paralyzing. As said before Darui's field can be overridden by a quicker auto-changer or a mystery field - and if you lose field advantage his effectiveness plummets. Mifune's buff, while a bit lower, is innate and not reliant on a field being up.

Darui seems to be more popular, and im guessing it's due to the show. Both of them are good ninja but depending on the cir*stance you use them, they can become underwhelming fast. I use Darui in my main team now, but only because im using a gimmick team around him. I would say if you could only pick one: Mifune for 1v1 events and arena while Darui should be for Ranked or GNW. Should be based on your preference.