More light is always better. 400 watts will always put out more lumens than 250.
Why not get more light? The reason a lot of folks opt for smaller lights is because they are cheaper to buy, they are cheaper to run, and they don't produce as much heat.
if you can handle the logistics of ventilation and heat, you are pretty much always best getting the biggest light you can afford. more light, more meds.
Thats why I'm going to 1000 watt HPS. yeah, I have to buy some extra fans, do some extra room prepr make sure I have a good AC, but I'm gonna get more bud/ft^2 without a doubt than I would with a 400 or 600.