If you're buying from a good-quality supplier like hellolights, AH supply, liveaquaria, or marine depot, then I'd go with dual 10k if you're buying from an el-cheapo distributor, go with the 10/20k mix
already got them..its a Current Sunpaq ballast running two 65w pcs.
i made out like a bandit today..our local reef society had a monthly meeting and i got the retro fit kit w/working bulbs for $30..the berlin skimmer for $10, a 48" 4x65w coralife unit with three working bulbs for $20, and two metal halide ballasts and fixtures for free! he gave them to me because he said when he first acquired them (used) the ballasts worked but theyve been stored in his garage for a few months and now when you plug them in they are hit and miss..either way, free is free..i can always open them up and try to find out whats wrong!