I come from the "Tim Allen" school of strobes, meaning the more power the better. But more important than raw power, is flexibility of distribution of that power. I have to admit I am out of the loop on current strobe systems, still using my big heavy Normans. My Norman's have 4000ws of power each, but the switching system and varible power control gives me enough flexibility to put 2000ws out of one head and 200ws out of another, using the same pack. Adding a "dump" head (a head that you point out the door, not onto the set) will split that 200ws and make the working head 100ws, and the varible power control will get it down to almost nothing.
Sometimes you need a lot of power, for example if you're using a softbox outside to light a group, or if you're inside and putting heavy gels over the lampheads, or if you need a lot of depth of field for a group shot. Even though you currently have a 15x15 space, you might rent something larger for a shoot, or go on location, or have and outdoor shoot. You should buy a system that will let you grow, and perhaps rent additional parts as you need them. In any case, I would opt for something with at least 400ws per head if you can.
Dynalite is a very popular system, (Playboy uses them) and one school of thought says that you should have several smaller powered packs for maximum control, which is what Playboy does. Also, the smaller packs are lighter and easier to travel with, and really adequate for most purposes.
I rememeber back in my portrait studio days (lasted about 8 months) that 400ws seemed like a lot of power. In a commercial studio, that's a hair light with gels. I did a shoot last year where I was able to put all 4000ws from one pack through a single head in a softbox for a shot on a roof, where the light was about 20' from the the models. We were lucky to get 5.6 out it on 100asa film.
Andy Pearlman Studio