I have had both, the XM provides better growth in my opinion,is definetely more yellow than the Coralvue Reeflux. Both of them have about the same intensity, but the Coralvue Reeflux 10K provides much more vibrant colors if you ask me, the blues are much better looking also. I am going to switch back to XM for the next round for growth, but will switch back to a better looking bulb for color. Aquaconnect 14K are an awesome bulb, but are very exspensive and hard to find.
The Aquaconnect 14k bulbs give the most par and best color of any SE bulb I've ever used. They are on the expensive side (around $110+) but hold their spectrum longer and give more PAR than many of the 10k bulbs. Hopefully, Iwasaki will come out with a 250w version of their 14k to give the ACs some competition. The Iwasaki 175w 14k bulbs are giving some of the 250w bulbs a run for their money in regards to PAR and the color is very similar to the AC 14k bulbs.
I had an XM 400 watt 10K. It was the crappiest couloured 10K I have seen. It made my water look like someone had peed in it, it was that yellow. I called XM and they said that they burn a little yellow to begin with. I tried it for 2 months and it never changed to a crisp white. I now use Reeflux bulbs and love them. They are inexpensive as well. You can check them out at reefexotics.com.
Bulb choice is largely a personal preference. People like different colors, so look at different setups with the bulbs you're thinking about.
As a general rule of thumb, the more blue the bulb is, the lower the PAR value, and thus the less growth you'll get out of your corals. However, the coral colors seem to "pop" more under blue light, so pick your poison.
Here's a quick chart. PAR values assume the same wattage lamp:
5500K ---- 6500K ------- 10,000K ------- 14,000K ---------- 20,0000K
orange yellow white blue tinted blue
high PAR good PAR lower PAR