Here's the 250 watt DE's I've used over the last year:
AB 10K's; hated them, very bright produced good growth, but way too yellow.
Coralvue Reeflux 10Ks; very bright, still just a tad bit too yellow for my tastes.
Phoenix 14K's; loved them in the beginning. Now at the 7 month mark and they're still okay.
Hamilton 14K; been using it about 4 weeks now and I like it. A nice white/blue light.
Replaced the Reeflux 10Ks with XM 20 K's today.
I love the colors that the 20Ks produce, but I wish they were a tad brighter. I'd bet that a 400 wat DE 20K XM would look very nice!
I do think that when my Phoenix bulbs are done, I'll be replacing them with 20K XMs for a while. If after a while they are just not bright enough for me, I'll go back to Hamilton 14K's.
Hope this helps some.