Korasi because you can use a dragon defender in your other hand; it increases your slash bonus and strength bonus which means that you can hit harder and more accurate.
Wouldn't the easiest solution be to check your stats with both of them, then consider that along with the speed/offhand slot difference? Personally I'd go with SS since AZs don't hit much anyway and your prayer bonus doesn't really matter.
wow. I'm extremely surprised you guys are all saying korsai. SS is the equivalent of a whip, but you can't use a defender. I'd take SS over korsai any day.
Take your pick I guess they aren't a whole lot different after all. Now I'm not entirely sure what would be best for zombies. would you rather have +12 attack +25 def bonuses or +6 str bonus?
You can use pray book as well, but that will lessen the attack advantage for the korsai
Korasi and dragon defender would be a much better choice than an SS. Personally, I don't find the SS is even that superior to a good one handed weapon with a dragon defender or an unholy book (depending on where you are training). I used one to get a few strength levels on my journey to 99 and it didn't seem to make a big difference in exp p/h.