For the Redeemer, you need 60% IAS to reach a 4fpa Zeal, whereas only like 10% for 4fpa with Lightsabre (I think).
A perfect Redeemer should have something like a 328 max damage, and a perfect Lightsaber has ~135.
328/4=82 82/2=41 maximum damage from WEAPON ONLY in PvP (assuming 50% Damage Reduction on Enemy).
135/4=33 33/2=16 maximum weapon damage. Now, factor in LS's 1-200 lightning 200/4=50 50/4=12 (assuming 75% lightning resist) and 120 magic dmg 120/4=30 (magic unresistable).
You get 16+12+30= 58 maximum weapon damage (Don't forget that possible Chain Lightning)
However, the Redeemer has a few charming qualities as +2 skills, -33% target defense, and LOW requirements (which means you can stack a bit more into VIT usually).
The +2 Skills will net you another 34% ED from Fanat, another 12% on Zeal, another 30% on Charge, and another 30% DEF on Holy Shield.
Not to mention the Attack Rating bonuses and whatnot.
Personally, I would go with the Redeemer, if you can get the IAS (unless you have a lot of expensive jewels to buff a Lightsaber).
Let's see if blckshdwdragon has a problem with this.
Edit: The +damage on Redeemer looks like it's applied to OVERALL damage, so it won't appear on the weapon (208 max dmg on weapon). The overall effect is still the same though.