What CFL wattage should I go with?

 
If I absolutely had to flower with cfl, I would use no more and no less than 8x 26w soft whites. Any more and you might as well get a 250w hps and quadruple your yield, any less and you'd be better off spending your money on a bag of weed. Also, 26w bulbs are 4/$12 at home depot so they're more economical than larger bulbs. I would consider scrog a must in this situation, cfl's are inefficient as it is and poor training methods on top of that can obliterate your yield.

 

For that space...either CFLs, or a small HPS, which would cost more than CFLs, and be about the same price as a 400.? Could look for a small dimmable.

Its twenty square feet in your area, take away one or two for pots and lights, so focus on lighting a 3x2x2 as best you can.

 

Well either or, depends on if your get a small MH/HPS and switch bulbs...just for this. efficiency of the blue spectrum. Also, with a small HPS/MH you can still add the opposite spectrum cfl in each corner towards the bottom or where ever. Even things out.

 

An HID light will pay for itself first grow, doesn't matter what size. I use a 250w in a 2x2x4, 5-6 zips consistently. If I went with 250w of cfl it might be cheaper up front but I'd be losing out on half that yield, so in the long run it costs more when you run out and have to go back to paying street prices till your next harvest comes through. $140 light grows me $2000 worth of pot at street value in my state, every grow. It just doesn't make sense to flower with cfl if you can use an HID.
 
Here's a few sites for you to browse
 
http://www.hydroponics.net/items/
 
http://www.htgsupply.com/Default
 
http://www.greners.com/
 
edit: you need a ballast for that bulb

 

 
You do realize that bulb won't work without the proper ballast, right?
 
It you have a space that small, you could just go with CFL's. They don't need to be fancy expensive ones, but if you can afford the larger 150W (actual usage 42W) you can get a socket that plugs right into a power bar, and a socket splitter. They are around $15 a bulb depending on where you live, but worth it IMO for the extra light. They sell 6500K which are good for vegging, and 2400K which are what you want for flowering. With such a small space you probably don't need a HPS. You really need to consider the extra heat with HID lighting as well.
 
Here's a small grow box I built from a dresser if you want to see what I did with this small space. http://forum.grasscity.com/absolute-beginners/1294750-stealth-grow-box-only-80.html