I voted RB, and here's why.
Comparing IOL to RB, the way I look at it, as they're both very scheme dependent for who you're looking for...and there's an imbalance there. If you bring in scheme-fit guys on the IOL, they're not as robust in pass blocking...nor are they built for a power run game.
If you bring in scheme-fit RBs, they're perfect for the confusing stuff and getting out on the edge, but they won't be successful at power running because the line can't support it.
Everything starts at the line (this is nothing new, but it gets forgotten often). A good line can make an average RB look good/great, and bad line can make a good RB look inept. Its far easier to replace an RB or two than it is to replace two good IOL.
That could fail miserably for all I know...that's just how I see it. It would require the team to shift some emphasis off of the ZBS I think to really have an impact.