Got confused. Thought the the name of the forcesaber was 'Mantle of the Force'.
So we have one Dark Side weapon and a general Force amplifier, unless I'm mistaken. (And if I am, I request you show civility and understanding when correcting me.) We should definitely sell the Forcesaber to the Jedi. We're not going to make use of it ourselves on account of its sanity damage, so why not? They'll pay handsomely if only to get it off the market, and Dooku Captures shows that the Jedi/Republic are willing to pay a lot of money just to keep a valuable asset out of the enemy's hands (assumed, actually, that they'll pay more to do that than the enemy would pay to keep that asset.)
As for the Mantle, so long as its benefits are significant, we should keep it in order to boost our precog. Otherwise, if it only does something minor like give +2 to Lore, sell it to the Jedi.