I had a 250W HQI pendant over a 15 gallon for a while (had a spare tank in the closet . . . and a spare light . . . hey, let's set up another tank!). Heat wasn't a problem as long as I kept the light a bit higher off the tank. I also had a clip on fan blowing across the surface of the water (open top).
As to brand of bulb, I had a Hamilton 14K first, now have a Phoenix 14K. I think I slightly preferred the Hamilton but I have always preferred a whiter light. Either is a good choice. I borrowed a 20K briefly to try it out and felt that it was too blue and dark.
My bulb choice was affected by the location of the tank as well--in my home office/computer room. It felt odd working in a room with 20K lighting. The 10K was actually quite pleasant in the middle of a dark Cleveland winter. 14K is tolerably blue.