I just got a D600 (wow) with 28-300 lens and realize I need to upgrade from my SB400 flash (used with D90 about 5-6 times). I and have two sellers on the line today. One has an SB700 and the other an SB900. I've heard all the nightmares about the SB900's overheating problems - and it's really quite large. I don't want to put more on my back than I need to... I'm 4'10" and about 95 lbs. Suggestions?
I can't speak for the SB900 but I have the SB700 which I'm happy with. I think Thom Hogan speaks well of the SB700 too. You probably know that you can use it in remote mode with the D600 on camera flash as the commander as long as there is line of sight to the SB700 receiver module. The SB900 probably works as well that way but with higher output power.
The SB-900 doesn't have an overheating problem. The problem is User perception. Speedlights were never designed to be fired continuously as fast as they can recycle. Check the owners manual on any Speedlight. The significantly faster recycling time of the SB-900 combined with a machine gun shooting style can result in themal shut-down. The SB-800 has a recycle time almost twice as long depending on the type of batteries being used. They can be fired continuously until they melt. I have seen it happen. The thermal shutdown feature on the SB-900 prevents the flash from being pushed to distruction.
The current SB-910 extends the recycling time as the Speedlight heats up to prevent the thermal shutdown from kicking in.
Both the SB-900 and SB-910 zoom out to 200mm compared to the SB-700's maximum range of 120mm.
It you don't plan on using flash beyond 120mm the SB-700 or even a refurbished or used SB-600 (max. Zoom 85mm) will suite your requirements very well. More so since you are looking for the smallest and lightest footprint.
If you need the additioan range and power of the SB-900 using common sense goes a long way in preventing thermal shutdown. Good Luck and Enjoy your Nikons!
I have both the sb600 and the sb800 and both are excellent IMO. Might be worth looking for a second hand one of these if cash is a concern.
If you're not totally tied to Nikon, then you may want to consider either the nissin or yongnuo ranges of flashes. Personally, I do not rate the nissins, but the yongnuo are rather good IMO and feel solidly built.
I picked up a brand new manual only flash for about £50 (for strobist stuff) and IIRC their entry level TTL compatible flash is only £60 - they do of course make higher spec'd models.