Comparing the EP300 to the SF200, there is only 50% more power (half a stop or 0.5EV). However, the extra cost is evident with the additional controls and features, such as proportional modelling lights (representing the power output of the flash setting etc), the audible beep can be turned off, the modelling lamp can be set to turn off during recycling (providing a silent "ready" confirmation), EP300 goes down to 1/32nd sec (SF200 goes down to 1/16th), EP300 tilt can be adjusted single handedly, and finally, the modifier mount on the EP300 is metal, whereas it's ABS (plastic) on the SF200.
Downside? EP300 costs £60 more per unit.
The SF200 will take pretty much any "S" fit accessory going, except the Fresnel spot. Having said that, I have found the Calumet speedring to be far too tight on the SF200, and I haven't been able to fit the Calumet Nova range of boxes (or any other Calumet box that utilises their speedrings for that matter) to an SF200 head. They attach to the EP300 without any problem.
Basically, it depends on whether you believe you need the additional features and benefits of the EP300, and if those features are worth the additional cost?
The SF200 are excellent heads, and I only took possession of them a few months ago to do a
comparison between studio and home environment portraiture. I was very impressed with them to be honest, and whilst I expected to stick with my EP600 and EP300 heads generally, I find I use the SF200 heads an awful lot for hair lights or spotting etc.
Oddly enough, the SF200 heads seem to get used a great deal within my product studio, as you tend not to need a vast amount of power due to the working distances involved.
Can't say you'll be disappointed in either head, to be honest.
As for situations where the SF failed to deliver, it's only happened where I needed more output, and in those circumstances I switched to the EP600 heads.
