Saver gives 100% bounce recovery. Assassin gives what? +20% base crit? Whatever it is, double that **** for back hits or something. I mean you can get Saver and make a crit build and then Assassin is not even worth being an option.
Assassin was recently reworked where the higher CRI you have, the higher multiplier it gives. For reference, around 500CRI is about 100% CRI rate for Assassin but is 50% for everybody else. There's a few scrolls released such as Volador that are pretty much anti-saver, so Saver isn't entirely the superior option now.