@Bulldog11 Something that maybe should be pointed out just in case your not aware (and to be clear). With electricity, we can make a simple analogy to help with understanding. Voltage is very similar to "pressure" (psi) with gasses or liquids and is considered a form of potential energy. Current (amps) on the other hand is more related to "volume", so a similar analogy with gasses or water would be CFM or GPM. In all actuality, amps are directly related to the total number of electrons that move past a given point in a given time.
The Volume (amps) that are delivered are based on the demand from the load (I.E. lights, a/c, whatever). If your load, for example, is a 60 watt bulb, then that 120v 30A line will be have only 0.54 Amps being drawn. So, it does not matter what the device is rated for, in terms of amps; provided it is less than your breaker max (technically 75% of max).
So, you can use the wire "as is" to run any device up to 30 amps, but you don't have to fully utilize it. Since there are no devices that can be rated for 30a @ 120v, you would have to split that line in order to use the full available power for that circuit. One advantage of changing to 220v would be that there ARE devices rated for 30a @ 220v and you would not have to split the line; while being able to take full advantage of available power.
Another advantage to 220v is related to power loss. The amount of power lost in a given line is based only on the volume (amps) running through that line vs its' resistance to that electrical movement (ohms). The less volume (amps) you have going through a line, the less power that will be lost to that line in the form of heat. Doubling the voltage, for a given power, will cut your amperage in half; this is why 220v can run more power (watts) with less volume (amps).
It's pretty much going to be up to what you need / want to hook up to that circuit. Do you just want / need 1500 watts or so? Then you can leave it as is and just run the device. Do you need 3000 watts but have devices rated for 110v? Then you can split your line to double 110v's with a MLO box. Do you want to run 3000w but have some HPS ballasts that can run on 220v? Then you would be more efficient to run a single 220v line vs double 110v circuits.