I have used many different kinds.
1st) Cool Touch Lighting 400w SE on a magnetic ballast. BIG BULKY and HOT
2nd) PFO 250w SE with XM 20k SE bulbs. Big, bulky, noisy ballast, but less heat. I liked the color, but was a bit blue
3rd) ARO electronic ballasts with XM20k SE. Quiet, energy efficient, and very small. No real cons that I can think of.
4th and favorite) ARO electronic ballast with 250w DE Coralvue 20k Bulbs. All of the above, virtually no heat, and the color is perfect IMO. Not too blue, not too white. I did try the 14k, but it looked like a 6500k in my tank. From what I understand, they dont fire properly off that particular ballst.