Well, between the SB700 and SB910, if money isn't an object... the SB910s are certainly better. I had SB700s at one point and have upgraded them all to SB910s.
910 advantages over 700: supports external power pack, higher zoom range on the heads, when used as a master, it supports remote control for 3 groups of flashes (700 only supports 2). Also has a stroboscope mode. The only disadvantages are cost and size. Size to me isn't a big deal... yeah, they're bigger than an SB700, but if you've got an SB700 mounted to anything... it's not exactly a lightweight setup anyway.
