Anything less than 120watt is just too little. You will be disappointed.
However, anything 300w+ should be pretty decent.
I believe
this to be the best LED light on the market... currently anyway. It only uses 1w and 3w LEDs... if you can hold out for the 5w LEDs (or make one yourself) you would be much better off. It is cheaper to make it yourself.
The 350w is comparable to a 600w hps and the 700w for maybe 2 600w hps. You should believe half of what is advertised about LEDs. I'm not sure what the 90w UFO is comparable to... nobody seems to like it though.