Blennies in general just have a great personality and are an awesome addition to a tank. Personally, I would choose one or the other, not both in that size tank.
The lawnmower blenny needs established rocks to pick on, they usually don't survive when introduced to a new tank.
I've owned a lawnmower, combtooth, and a tailspot blenny, all had very similar personalities (my favorite is the tailspot), so the choice is really up to you. They can all be territorial, but the midas is usually the most aggressive. Midas blennies also have been known to pick on gobies, firefish and mandarins, just keep that in mind.