MH over a 20 gallon...

[midgetwaiter]

That simplifies things then. Somebody with more SPS experience may correct me on this but I think you could go with either unit and be happy.

The 250 would give you the option to put your high demand critters almost anywhere where with the 150 would still require a bit more planning and be less flexible. Really fussy stuff would still want to be high and centered. On the downside with the 250 you'd have to be careful with acclimating new additions and some things like zoas and mushrooms would be difficult to place.