Which will use less electricity: a commercial 5 ton 3 phase 13 SEER unit or a 5 ton single phase residential 20+ SEER unit? I am trying to figure out what type unit to put in our restaurant bar.

I would go with a 3 phase because of the advantages of 3 phase over single phase. Three phase won't require as large of wire size as a single phase unit to install, three phase won't have a capacitor on the compressor, and they usually start easier. There's not a real big savings on energy, you draw less amperage but it's on three wires instead of two so the actual KW usage is close. A 20 SEER over a 13 SEER will save you a little energy, but I like 3 phase.

Three phase
Demand meter required for business in this area. Bad news for electric bill.
High minimum monthly usage requirements also for three phase in this area.
Check with your elecrical utility about this before you jump. I also like 3 phase equiptment.

Good Point... check with your electric company, I was shocked when I moved South and found out there is "NO" demand meter on three phase. If you already have three phase it won't make much difference.