If you do only headshots, you can work with a fairly low-powered flash head or system because the head(s) will be quite close to the subject. If you light large subjects or areas, you’ll want a more powerful system. Flash heads are often rated in watt-seconds (w/s), or in joules in Europe, but watt-seconds aren’t a measure of output; they’re a measure of power. Current monolights range from under 200 w/s to over 1300 w/s. All other things being equal, two systems of equal power should produce equal output. But all other things aren’t always equal—flash tube, cable, the nature of the capacitor, reflector and more enter the equation. A better indicator of flash power is estimated output (usually given as the aperture required at a specific distance, often 10 feet), when available. If you can try the system (in the store, or by renting it for a day, or via a friend who has it), you can determine this yourself with a flash meter.
Besides maximum power, you should consider the ability to adjust the power of the flash burst. It’s easier, in a multi-light setup, to adjust the power of each flash head appropriately than it is to physically move each unit closer to or farther from the subject to establish the desired lighting ratio. Lower-cost units might operate only at full power, or at full and half power. Better units might let you adjust power down to 1/32 power or less, in full-stop or fractional-stop increments, or even continuously.
With some flash systems, the color of the light changes with the power. Shooting RAW with digital cameras, you can deal with this in postproduction, but it’s best to get a system that minimizes color shift with power setting. Some manufacturers provide data on this, others don’t.
Better (generally, higher-priced) monolights have better repeatabi-lity—each flash at a given power setting is just like the last one. Lesser units might provide slightly stronger or weaker bursts from flash to flash.
Camera-mount auto-flash units have very short durations—generally, from around 1/1000 second at full power, down to 1/20,000 or so when used at very close range or set to minimum power in manual mode. Studio flash units have slower durations— often starting much longer than 1/1000 and rarely going above 1/2000. This isn’t a major consideration, unless you want to do those "frozen-milk-drop" kind of studies, in which case a camera-mount unit is a better choice than a studio unit.
Because the flash burst is very brief, you can’t study your lighting as you can with continuous "hot" lights. So, most studio flash systems provide modeling lamps, which are continuous-running bulbs adjacent to the flash tubes that let you see what the lighting looks like. Unless you work in a very dim environment, you’ll want a system with powerful modeling lamps. In multiple-light setups, it’s also helpful if the modeling lamps are proportional; that is, if they automatically brighten or dim to match the power setting you use for each flash head. With some systems, you can manually adjust the brightness of the modeling lamps.