I'll start out by saying I never limit myself to one weapon. But if I had limited resources, then I guess I'd have no choice but to pick one.
A kryss is really nice in PvP because of Infectious Strike. Poisoning can make the difference every once in awhile, though there are many ways to be cured very quickly.
A Leaf Blade does quite a bit more Damager per Second, though it's primary attack (feint) is barely worth the mana.
Both the Kryss and the Leaf Blade have Armor Ignore which is very nice to have.
All that being said, I'd take the Leaf Blade for the higher damage, but I'd still try to have a poisonous Kryss or dagger around for when the need arises.