I am shopping around for a 400W SE setup and here's the info I gathered:
The Radium20K, XM 20K and AquaConnect 14K bulbs seem to be the most popular.
The Radium has a bit more PAR than the XM but is said to have a very short lifespan of about 5-6 months where the xm is good for 9-12...both bulbs are similar in appearance (I think the radium is slightly more purple) and cost pretty much the same...
The Aquaconnect has ALOT more PAR than both the others (170 vs 100-120) ans is consequently brighter than the others and supposedly makes your corals POP out more, in the sense the the colors are more intense...
The downside with the Aquaconnect is the cost, which is almost twice as much as the other 2. It supposedly lasts longer though maybe 11-14 months. I think I will try one of these and the XM 20K to compare...
Note that these bulbs will look better on HQI ballasts, as they will be brighter and give out more PAR than on any other type of ballast...
hope this helps...