It depends on what you want.. Both have great skill set and talents,
Scythe can diminish people w/ Resurrect + Martyrdom, not to mention Sandstorm + Massive Boulder..
Cygness can do three straight Water Torrent w/ Rain Prayer, SPD Boost can give you first attack chance w/c helps w/ PoD and lastly Sniper + Ambush might possibly be x3 or x4 critical i'm not sure..
It all boils down on what books you have and what you really want for a strategy.. Static or Dynamic?
In terms of balance Cygness is pretty much even on almost all stats.. W/c gives him an all-rounder advantage

Cygness offers build flexibility and team synergy
Scythe however is geared towards relentless attacks and dismissing as much damage as possible

the only faster monsters than Scythe are Rhor, Ignightmare and Felicity.. Two of w/c can be OHKO w/ Massive Boulder and both wouldn't dare attack Scythe and would most likely be switched at that instant..