150W or 250W MH?

[digital-audiophile]

Howdy folks,

Starting to set up a cube I picked up on Boxing day. 20 long x 18 Wide x 24 tall.

There is about 21" from the sandbed to the top of the water. I am debating if I should go with a 150W HQI MH or a 250W HQI MH.

I want to venture into SPS and clams and I am wondering if I get the water penetration with the 150 and avoid some of the heat issues if I were to go with the 250.

Thanks for any advice you can offer.

Greg