the big reason to go with the 250 over the 400 is heat. you don't need the 400 as they are really better at punching through to deeper tanks. Also, having a great reflector will put more light into the tank, and the 250 will do just fine. the 400 would just add unneeded heat to the tank, and draw more power than you need. Sure, a 400 would get the job done...but you just don't need to pay all of the extra's (extra $ for the bulbs and ballast, more fans/bigger chiller, and more every month in electricity.)
the VHO's: it really depends on the look you like. what coloration do you want to see?