Depends on what you're replacing. If you have 2x55 (110w) and replace it with a 150w MH then you'll be using 40 more watts. If you have 4x55 (220w) and replaced it with the 150w MH you'd be using 70 less watts.
My plan is to one day replace the four 1x96w CF AquaLights over my 125g tank with a pair of 4x39w Tek lights. That'll drop my power consumption by 72w and look so much sexier.
Yeah, look at the wattage numbers for figuring this problem out, not what 'type' of lighting it is. The ballasts will make some difference, but when comparing ballasts if thier all electronic there all going to be close. The Icecap ballasts are just about the most efficient one out there though (for MH, not familiar how they rank with flourescent). I dont know if it will make much difference but MH often will raise the temp of your tank, which could mean you heaters might not have to run as much, saving you power perhaps.