well, some bulbs have higher efficiencies measured in Lumens/watt. And it's a lot harder to build a box with 100 2 watt bulbs rather than one 200 Watt bulbs.

You should consider the cost of buying all the extra fixtures too. I think having medium sized bulbs say 85Watt would be the best as you can spread them out so all plants get good light, not just the ones under the bulb (if you had one big one)