I've got all kinds of light making devices. I've got a couple Alien Bees, a B400 and a B800. As well as the big White Lightning X3200 which I bought to use with Polaroid 55 on my 4x5 before it was discontinued since it's negative is rated around ISO 25. On top of all of these plug in strobes I've got a couple of Canon Speedlites, a 550EX and 580EXII which I've been using lately on indoor editorial shoots. THEN, I've also got a ProFoto AcuteB which is a 600ws battery powered strobe for use when you don't have a wall outlet.
So basically I've got every option from a little tap of light to a huge 'you just shorted out the neighborhood' POW! However I often find myself questioning which I should bring or use in any particular situation. Lately I've been using the speedlites with the wireless controller more in a quasi TTL mode where I'm playing with flash exposure compensation on the flashes themselves. I know that the infrared controller can do ratios between flashes, but I've found that it rarely actually does what I want it to. I could also use them in manual mode with pocket wizards like they suggest on Strobist, but for some reason, I can't get myself to use them that way. If I'm using little flashes, I want it to be more automatic, if I want a pain in the ass manual setup, then I feel like I'll use big guys. Maybe that's short sighted.
Also with the speedlites, I never get the same quality of light as I do with the big guys. The way they're shaped and how they sit and point into the umbrella from their bracket doesn't seem to give the same spread as a centered studio strobe would. I haven't done any definitive experiments, and maybe there are better brackets out there, but the one I've got just doesn't do it for me. The light source ends up skewed way to one side if you're using any reasonably sized umbrella, and I can't imagine you get a nice even light that way.
Anyway, on tues I went and shot my delightful friend Tia at her restaurant. It's a big space with high ceilings and I was going to use a big 60" softlighter, so I thought I needed a big strobe. The power plug on my B800 is cracked (keep meaning to get it fixed) and I didn't think the B400 would be enough, so I borrowed one of Meg's B1600. Well, long story short, it was way too much light and I ended up with it almost all the way down the whole time. The moral of the story is that the B400 would have been fine, if not even less. It's one of those things where if you're there and you don't have enough power for what you want to do, you're screwed, so you pack big and then you end up with too much.
As for the ProFoto, I haven't used it nearly as much as I would have liked or would have though. In fact I need to make a point of using it more. To that goal, I brought it up on the roof with me when I was shooting Brie last week (see post below) along with the small 30 something inch softlighter. It's not the most powerful thing out there and if you're fighting the sun, you've got to know your limits. Last year at one point I tried it into the 60" softlighter in afternoon sun and at full power it didn't have quite enough oomph to get the job done. You see, if you're using a big strobe outside in the daytime with a digital SLR you're limited when it comes to your shutter speed. Most of these cameras will only sync with an external strobe up to about 1/200th of a second (The reason why is a long explanation that I'll just link out to instead of reinventing the wheel http://dptnt.com/2007/10/flash-sync-speed/) The practical upshot of this relatively slow shutter speed is that even at iso 100 your aperture is going to have to be stopped way down to get the exposure right, let's say something like f/13 or so. So now your strobe has to put out enough light to handle f/13 with the modifiers and distances you're working with. And that's only to have the strobe equal the sun. If you want to pull down the ambient, you're looking at f/16 or f/22 into a big softbox, and that requires a lot of power. This is why people use giant generators and 2400w/s packs in those big outdoor shoots people like Annie do.
As a quick aside, Pocket Wizards just released their latest triggers that do some timing magic to get some cameras to sync at higher speeds, up to something like 1/500th of a second max. This would allow you to use a wider aperture and theoretically need less power on your strobe. The thing is that really it only buys you about a stop of light, and from what I've read it can cost a little power on the light because of the way it fires the strobe slightly early in order for it to line up. So as far as power goes it might be a wash, but I need to do some more research on it.
The other option outside is to use a speedlight on high-speed- sync. Basically the light emits an even low-power buzz of light that is on while the shutter is open, thus making it work at any shutter speed. I personally haven't tried it through any softening modifiers in afternoon sunlight, so I don't know if this is the answer, but you could for example throw the shutter speed up to 1/4000th and open up the lens to f/3.2 or so. You'd end up with a blurred background while still using an additional light.
One other idea that some people forget is just using the modeling light from your strobes at open apertures. Sometimes I do this when I want a really thin plane of focus that gives the effect of something like a large-format close-up. Just get a fast prime and shoot at f/1.4 in Av with the modeling lights. The only real problem I have with this is that I need to get or make some strip boxes to get the effect I'm really after.
As you can see, lots of options and lots of lights, but they all fill some kind of niche. It's like different wrenches, they all look similar, but they've all got different jobs to do. Please comment and add your own tip and experiences.