Power is very important to me because:
- if I'm using studio flash over hotshoe guns, I'm doing so for one main reason - power. Plain and simple. I often want as much power as that thing will spit out.
- you can turn a more powerful gun down, you can't turn a less powerful one up.
- if I'm shooting kids on seamless, or a three layer family group at their home (at a wedding for eg kids on floor, bride on chair, mum and dad behind), I want every single bit of aperture I can get at a nice ISO. Kids move around, so focussing is less critical. Three layers of people needs a decent aperture at moderate long focal lengths, if I can get f11 out of a 400 watt/sec, you'll be running at f8 out of your 200.
- the more powerful guns will never leave me thinking, humm... should I upgrade? 400watt/sec is a nice happy medium powered gun. I regard 200 as a low powered gun.
- pump up the ISO is one way of making weak guns more powerful, but I can always make my more powerful guns even more powerful. 100-200 iso, not very noticeable, 400-800 noticeable on virtually any camera, imho.
- Changing ISO is a global change, affecting ambient as well as flash. Turn up the flash and leave the ISO alone only affects flash. Chaging ISO to compensate for an underpowered flash only works if 100% of your light is flash, and there is no ambient - i.e. inside a dark studio. Almost all my work is location to try and get natural photographs that may be flash lit, but look natural in a natural setting.
- there isn't much difference in price between a 200 and 400 - eg, its only £30 for the Calumet Genesis and £40 for the Elinchrom D-Lite. There is no real reason not to get the more powerful one then.
- many of my nicest modifiers are light suckers.
- my skys will be half as bright as yours, if you are trying to reduce a very bright ambient.
- easier to ratio with more than one gun - eg, if you have one gun that is twice as powerful as the other, then your main and fill are already set. Therefore....
- a 200watt sec gun would be useful as a second light, but not as the one and only light (if that's all you can afford) and definitely not as a set of two or three equally powered guns, which I cant see the point of.
All in all, 200watt sec would have me reaching for my cheque book sooner rather than later.
YMMV.