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Don't hold back on flash power because once you start diffusing and/or bouncing light you run out of light very quickly.
Make sure that whatever you get can act as a master unit and as a slave unit in a multi-flash setup.
Get one that has a good menu system or preferably suitable direct control buttons for changing modes, etc., that might otherwise require you to rummage through your camera menu system.
Get an off-camera flash cord so that you can have your flash a bit farther away from the camera to reduce that flat look that direct flash often produces, but make sure it isn't too short to use comfortably. You might want a flip bracket of some sort too.
The more flexibility you have with one flash unit the longer you can avoid getting a second one and an assistant.
The SB-700 and later let you swivel the head left and right up to 180 degrees. This is far nicer in practice than one which goes 90 degrees one way and 270 degrees the other way. Every time you change your camera orientation you will need to re-aim your bounce flash or reposition your flash for correct shadow location, and an appropriate bracket can help with that.
Make sure that the overheat protection doesn't leave you stuck with no flash for an extended period. Newer models will gradually slow down the shooting rate before they stop completely whereas the older ones had no thermal protection at all or else ran at full rate and then suddenly stopped dead for 10 or 15 minutes while they cooled down.
A good flash is like a good tripod in that it shows how poor a poor one can really be, and a good one will probably outlast your cameras.
Be prepared to spend a while becoming familiar with the system. Flash seems to have a mind of its own when not used manually, especially with auto-ISO on the camera. Different shooting / metering modes can give very different and perhaps unexpected behaviour, although there's bound to be logic in there somewhere.