This is a tough call for me.
My initial thought is Spencer, because we so desperately need someone who can actually put the ball through the rim. He also brings a lot of other things that fill the stat sheet. But if you look at his scoring last year, he was assisted on 62% of his shots (61.2% this year so far). We don't really have anyone who can consistently get him the ball on the roster right now, and he'd be the immediate focal point of every defense we face because we have no other shooters/scorers.
On the flip side, we desperately need a distributor who can get the ball in the hands of players in a position to score and makes the team better around him - which Mulcahy brings. But he himself is hesitant to shoot and isn't really putting points on the board himself.
We'd be much better if we had both back, but if I had to pick only one.... I'd probably hedge very slightly to Spencer.