Here are two Caster 100w Metal Halide ballasts, i wanted to see how different these two ballasts are. The MH100 (left) is the more common, cheaper ballast with only one voltage input (120v), this ballast is common in Casters residential grade fixtures. This ballast comes pre-equipped with a Caster IG-240 Ignitor to start the lamp. The ballast core size is 72x98x32.6mm.

The MH100-4 (right) is the less common commercial grade ballast with four voltage inputs (120v, 208v, 240, and 277v), this ballast is often seen in Caster commercial grade fixtures, this ballast comes pre-equipped with a Caster IG-240 Ignitor to start the lamp and a 10uf MPP capacitor for power factor correction. The ballast core size is 72x98x39mm.

Now for the testing, both were tested with the same lamps, the line voltage was approximately 118v during the duration of the tests.

The MH100 current with a 100w metal halide lamp,
Watts: 111
Amps: 2.17
PF: .43

The MH100-4 Current with a 100w metal halide lamp,
Watts: 118
Amps: 1.17
PF: .86

Now i was going to try to get current readings from the lamp side of the ballast on the MH tests, but i feared the ignitor would damage my multimeter, so instead, you'll have to settle with ones from the MV test.

The MH100 current with a 100w mercury vapor lamp,
Watts: 118
Amps: 1.74
PF: .57
Lamp Current: 830mA

The MH100-4 current with a 100w mercury vapor lamp,
Watts: 138
Amps: 1.48
PF: .79
Lamp Current: 920mA

So looking at the current tables, it looks like the MH100 is optimized for 100w MV lamps, but not as much for 100w MH, whereas it's the other way around for the MH100-4.

The lamps used for this test were a Polerlites 100w MH and a Chinese Westinghouse 100w MV.