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?