250 or 400 watts?

[canadawest]

With a tank only 18" deep, and factoring in a couple inches of substrate, you'll likely be fine with 250W MH. You'll save on both the initial cost, and cheaper to operate, and much cooler to run.

If you are going to go mostly SPS, Clams, or other light hungry animals, and will have adequate cooling, then the 400W is not much more expensive.

Mostly a judgement call, but I think you'd be fine with 250W.