can't decide between speed light

[KmH]

I've been researching about speedlights/flashes a lot and if you use the SB600 off camera with a radio trigger, it can't be used with TTL. You would need the SU-800 to use the TTL mode, so any flash would work.

The SU-800 is optical IR only. (IR = Infrared)

There are radio triggers that can TTL , but they are not inexpensive. See the Radio Poppers and Pocket Wizards radio triggers. Pocketwizard® - Wireless transmitters and receivers