It's really up to you on either a Blue Jaw or Sargassum. IMHO it's really just a color variation based on a couple thousand (maybe just hundreds
) years of evolutoin in different locations (Pacific vs. Atlantic).
That said, Sargassums' are usually only available in one size, about 3" to 4 1/2", where Blue Jaw's run the gambit from 2" to 7", at least one's I've seen recently. Though about a 3 1/2" specimen is about the absolute best size, hardy enough for aquarium life, but not too large to have troubles adapting. I've had both, currently have a Sargassum, and they had very similar presonailites, a bit care-free, after the initial 'adjustment' period. Either one should be completely reef safe.