Which is better - 30 AMP 220V circuit or 60 AMP 120V circuit

You are getting the same amount of 'power' either way (120V x 60A = 240V x 30A), so from an electrical "power" standpoint it is a wash--either one will support the same amount of equipment in the rack...

You should do some research to be certain, but most modern computer equipment is equiped with switching power supplies that accommodate a wide range of voltages (manufacurers don't need to stock a separate power supply for each country; instead, they simply ship the appropriate power cord for the destination country). Given that, your equipment most likely will not care whether they have a 120V or 208V or 240V input--so long as you have the appropriate power cord to connect the system to the power distribution module.

My understanding from engineers at colocation facilities I've worked with is that most switching power supplies are a bit more efficient with a high voltage input (e.g. they produce less heat, which is wasted energy) which can save a bit on both power air conditioning load. The wire gague required to carry the same power at 240V is smaller and more flexible than the heavier wire required at 120V, which may be a convenience when routing cables within the rack.

I would confirm that all of your equipment is capable of accepting high-voltage power; if so, obtain the appropriate power distribution module and cordage to connect your equipment to the 240V 30A circuit.

Remember, too, that a 30A circuit should not be loaded up with 30A of equipment--you need to leave some "head room" to ensure that the circuit is not overloaded (I believe the rule is to stay below 80% of the maximum rated capacity (e.g. maximum 16A continuous load on a 20A circuit; no more than 24A continuous load on a 30A circuit).

Practically these are roughly equivalent services: 6.6KW (which is a LOT of power - I start seeing heat problems above 3.5KW in my racks), which is why the price is the same. As a co-lo customer this is all you really care about and the rest of this is all "datacenter management" stuff.

Heat and efficiency wise, 220V service is "better" -- Power supplies tend to have a better efficiency at the higher voltage (you may be able to squeeze one more server into the supply) & run slightly cooler.

If you are mounting an in-rack UPS system you can probably cut heat dissipation by 1/4 to 1/3 by using 220V power.

If your datacenter is not supplying your PDU you will have an easier time finding 220V equipment.

The real savings with 220V power comes from inductive loads (motors: Air conditioning), which if you're co-locating you don't care about.

My Bottom line recommendation: Go with 220V unless you have a bunch of 110V-only equipment.