Unless you are after the industrial look I'd box it as well. Trying to get a perfect neat tight joint around it might be a bit difficult depending on access. BTW is it an RSJ or a box section? (I assumed RSJ, perhaps incorrectly). Dominos would give you a more rigid joint. Remember that with either Dominos or biscuits you will either need to have sash cramps long enough to pull up the joint whilst the glue sets and that end grain to end grain glue doesn't always make for a firm joint, or you will need to install "dogbone" fasteners (kitchen worktop fasteners) on the underside of the joint to pull it up, which means having clear access to them from beneath