I used to use daggers with lyssa but have had a change of heart for four reasons;
1. Using a scythe allows me to use conjure frost for MORE extra damage
2. Aura of holy might.
3. Eremite's and mystic. Yes, daggers attack faster, but the two fast activating scythe attacks do a much better job allowing you to nail your window of opportunity than the faster attack rate of a dagger or a warrior weapon with IAS. What's better synergy with lyssa's than fast hits? On-demand hits!

4. Radiant scythe. It has wonderful synergy with lyssa's energy bonus, practically garunteeing +50 damage every time. And the best part is that it gives you a really powerful attack even if your foes are so inconsiderate as to not activate skills.