I think Jonathan has a good point at least in that pure isometric should not be considered better by default just because it's more 'realistic' (etc.).
I agree, and I furthermore suspect that if you want to run SS alongside DF in real-time, non-isometric icons would be less jarring, since they wouldn't be moving backwards. However, if you're not planning to run it while playing but only use it to check out the fortress while paused, isometric creatures that seem to fit in the game grid might be less jarring. As with the whole ASCII vs. graphics debate in DF proper, it's probably going to be highly subjective.
At this point, since progress on the SS art will only continue as long as the artists here want to keep working on it, I'd encourage people to just keep doing whatever they feel like doing, in whatever style they want to experiment with. If you end up with a couple of different styles, that might be a good thing - trial and error will decide what works best for the "official"/default style, and anything else could grow into it's own custom thing.
(Of course, if the primary owners of this project want things to progress in a specific direction for now, in order to get a more-or-less coherent base set up and running, now might be a good idea to mention it.
Btw, long-time lurker, first-time poster. This thing is pretty awesome and awesomely pretty. Keep it up.