I'm not really sure just what the First Evil's plan was for wanting to escape the Hellmouth. If it was, indeed, the very personification of evil, then it's influence wasn't contained in any way, and, since it was intangible, it didn't pose any more of a threat to me one way or another. It did mention once that it desired to be "made flesh," but I never got any real impression that that would occur simply by leaving the confines of its prison.
Jasmine, om the other hand, seemed to me to pose pretty much the same threat that the altered atmosphere of Miranda did in "Serenity," where everyone just became lost in a cloud of euphoria, except, of course, for those people who became her own special snacks.
At the worst, the First Evil was a threat to mankind only in a physical sense. Jasmine was a threat to the very spirit that makes humans unique from animals.
So. Jasmine, to me, posed the greater threat.