As you have probably figured out by the replies above, there are 2 factors to whether you need 30 or 50 Amps. 1 has to do with the type of service your unit can handle and the second is how much power you need to do the things you typically do simultaneously.
Dr4Film provides pictures above of 30 and 50 Amp plugs. If your unit only has a 30 Amp plug, than the answer is very easy since you can't use more than 30 any ways... If your unit has a 50 Amp plug on the other hand, then you can choose 30/50 based on your energy needs (see below), since you can buy a 50 to 30 Amp pug adapter at almost any Wal-Mart or camping store.
For calculating how much energy you need, you can use the list below as a quick reference to the power requirements of typical appliances in the RV.
A/C (12-15 Amps per unit)
convection oven (11-13 Amps)
refrigerator in 120V mode (5-8 Amps)
Space Heater (8-13 Amps)
Television (.5-4 Amps)
Toaster (7-10 Amps)
Vacuum-handheld (2-6 Amps)
Now for us, we have found that 30 Amps is almost sufficient even though we have a 50 Amp plug. We only have 1 A/C unit so unless it is very hot (and we need to run the oven/A/C and say TV al at the same time, we are fine on 30. In places where it is very hot though, we tend to splurge for the 50 if available so we can keep the A/C and a fan on no matter whether we are cooking or not