A friend of mine used XM 20k's on his outer orbit. They seemed a little low on PAR and lacking in spectral plot. It just gave the tank a dim blue look; IMO, the current ballast isn't the best for the XM's. Shortly afterward, he switched to the 14k Phoenix's; the difference was pretty incredible. Definitely seemed like a good choice for that fixture; it was brighter and more full in color. One great thing about Phoenix's is how they seem to keep descent PAR, even after 6 months. If I remember correctly the PAR readings from 6 months to a year weren't very different.
Both are great bulbs; like mentioned before, it really comes down to your ballast when choosing your bulb. I've seen XM's look bright and full with great color, but I think the Phoenix is the better choice for your light. On the side of comfort, even if you find yourself not satisfied with your choice, you can always compensate with your T5's.