I was just readin a study regarding PAR and MH's the XM 10 outperformed most other bulbs, except for the Iwasaki.
I have a 175 Ushio 10K on a 15, the color is beautiful, crisp whitish blue. On my 55 I have 250 AB 13 K's, and they are more white than the 10 k.
The 20 K's have alot les PAR than most of the other K ratings. Too blue for my tastes, but they are becoming popular.
Many have reported problems with Sunburst bulbs. However, I have never used them. I am currently using 3x 175 watt 20K XMs and they look great. They are blue, but not as blue as I expected. It reminds me of what it looks like when snorkeling in Puerto Rico(like a 10-20 foot depth). Plus, the corals flouresce better under the blue light IMO.
The sunburst have enough first hand accounts of problems to simply make me shy away from them. Same with high K Coralvues.
My view of MH's is stay with 10ks of a known and reputed brand. Personally I don't like the deep blue of the high K types, but that's a personal preference.
I used to run dual sunburst...and I really liked them. I did not use any other lighting in conjunction. The will start off more on the blue side, but will leave once they burn in.