I have researched the crap out of this subject as well and there doesnt seem to be a clear winner... to answer your original question though i would say just get a new ballast instead of investing a ton on a totally new system.
You could get T5HO's in combination with MH like reefer rob has suggested just keep the single working MH you have and get a 4 bulb
retro kit this is the best price i could find with free shipping in canada just be sure to select the right size.
Cheers,
Ken