honestly, i feel that its always better to get a bigger unit so that u wont have to overwork your chiller. Thus it will go easy on your elec bill.
Just like what LightingStrike said, it very much depends on those points that he mentioned. But most impt of all, what lights are u using ?
Im using MH250 + 4 x t5 and im using the cl650 and i still think its not good enough