I went through the same debate a while back (for my 150W) and ultimately stayed with the Pheonix 14K. There are some differences between the 150W and 250W bulbs, so take my input with a grain of salt.
The Ushio 14K definitely has more punch to it, tends to be a bit brighter and I would say is 'truer' to what natural sunlight looks like. I remember that the Ushio's give a slightly higher PAR (I remember reading somewhere that it is about 15% higher than the Pheonix bulbs) and consequently give slightly better growth. All that being said, much of those traits stem from the fact that they are very 'yellow' looking. I found the Ushio 14K to be yellower than my Coralife 10K it was replacing! I didn't keep them long enough to really comment on how well things grow under them but I would say for growth (based on PAR) and the look of a very shallow reef, use the Ushio's.
When I switched to the Pheonix, the colours absolutely 'popped'! Initially it almost looked surreal but as the lamp burned in a bit more (or my eyes burned in... either way...) it really started to impress me. Pinks looked PINK (where as they looked brown/yellow-ish under the Ushio) and greens went absolutley electric. I have had great growth with the Pheonix (way better than my ancient Coralife 10K) and people are blown away by the colours when they see the tank.
Obviously, I am a Pheonix fan over the Ushio but like I said, I'm using the 150W and there are some differences between the bulbs in that respect. I would also recommend the Ushio 20K; a friend of mine had this one and it looked quite stellar (quite similar to the Pheonix in terms of colour). I suggest getting science-y and popping over to
Sanjay Josji's website. This guy really nerds it up when it comes to lighting and there is a lot of info on there to poke through.
Happy reefin'!