Applications... does it matter? Anything, really. Interiors, portraits, and outdoor applications. I don't know what I'd end up using it for frankly. The SB-600 is a good option, but the SB-400 is a really good-looking small and powerful package that I wish could be used wirelessly.
The SU-4 is an option, I guess, but I don't understand how it works. How's it different from the SB-600?
We have an R1 unit at work, and those SB-R200s are really nice but have much less power than the SB-400 (GN 33' vs GN 98').
Yes, it does matter. If you wanted to just light up some tabletop still life shots...the macro lights would do...if you want to light up a portrait, you need a different tool.
The Su-4 just tells the flash when to begin firing and when to shut off. If you use The D200 to command a Sb600, you can do so with iTTL, and adjust the output incrementally. You can go full manual and make adjustments to the flash from the camera without touching the flash unit. That is not possible with a SU-4 unit.
I would recommend the SB600 for you...that wireless TTL is really great for just starting out...
You might get to the point where you're shooting all of your flashes in manual anyway, but when you're just starting out, TTL is a great asset...go with the SB600...it's really fun and works like a dream...
Yeah, I think that's what I'm going to do. I was woo'ed by the diminutive size of the SB-400, though. Like I said, I have access to an R1 kit at work... I might have to borrow it sometime for macros, and maybe later on get one of my very own.