Personally, I think getting two a350 is a better option. You have the ability to tune it down to your desire intensity. I feel that the a150 are too limited in it's abilities. Also, if you ever plan to upgrade, the a350 can still be part of it.
For what it's worth, I'm getting two A350 for my 30x30x24 tank. I was told that one A350 would be enough, but I don't like having shadows.