When picking an AC unit, running cost is not the primary concern: sizing is the primary concern. If you pick too large or small of a unit you will have issues. Too small won't be able to cool, too large generally won't be able to bring humidity down (outside of the newer inverter units that can run slower for longer).
Tons (or, BTUs of cooling) makes more sense, because the efficiency for a given cooling amount is variable.
For efficiency, we have efficiency ratings in the SEER/SEER2 ratings.
