It really depends on who you're using in your team and your build. If you're going for a leech build, then Mifune would be better because he can deal Tai damage with his mystery+chase and can leech with them. Darui on the other hand can benefit from the lightning attribute boost that MB has, but is more geared towards Nin with only his standard attack as pure Tai.

Something to note is that Darui's passive barrier can be canceled with an activation barrier and other passive barriers if he's slower than other passive barrier ninjas... which may lead his passive to be kind of useless for PVP. For Ranked battle though, if you're heavily build with Water and Lightning then Darui would complement the team well until an activation gets put up. Another thing to note is that Mifune can dispel buffs and shields, which is pretty useful if they're giving you problems.

I would go with the one that's cheaper, but I would prefer to have Mifune because Sasuke can create barriers for a lightning team again and again unlike Darui. I got Darui, but at the moment he's outclassed by Sasuke for the team MB, Sasuke, Kimimaro, Iruka/Jugo.