If you're going to play a Rit I really wouldn't recommend running ST yourself. It's mind numbingly dull. It would be better to run something else yourself and delegate ST to a hero. An interesting idea might be build around Assassin's Promise rather than use ST itself but I doubt that'd end up working as well.
Anyway, to actually answer your question, I'd say context comes into it a little. Which areas and which difficulty? Areas with tons of anti-weapon stuff will favour the ST, for example. Context aside, I'd probably favour a permanent SY over the ST by nature of the SY bar putting out damage - something you certainly shouldn't neglect. Haven't really done any meaningful tests on the matter though.
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