The best replacement for sb 600 is sb 400.

The best alternatives to sb 600 are rated as following:
★★★★★  sb 400
★★★★  sb 700
★★★★  sb 800
★★★  800
★★★  sb 900
★★  900
★★  yn 560
★★  nikon sb 800
★★  sb 910
★  sb 26
★  sb 80dx
★  sb 28
  nissin di622
  sb800
  di622

Your alternative for sb 600 is not listed? Suggest!
Speaking about sb 600, one probably shouldn't forget about d80.

Other search terms related to sb 600 and their ratings are:
★★★★★ d80
★★★ d40
★ nikon d50
★ sb 800
You have in mind a companion for sb 600 and it is not yet here? Propose!



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sb 600
sb 400
sb 700