I'm kind of surprised that noone's hit the nail on the head here yet.
both scythe and GS are going to parse similarly on merit level mobs UNTILL you start feeding them atk buffs. once you pump enough atk into scythe, through outside buffs, you get a huge dmg boost from guillotine because of it being a milt-hit attack.
taking both to the HNM scene GS will generally parse better. feeding both enough buffs will put them on similar grounds, and in some cases scythe will do better.
I could probably fish up the math on this if you guys really want, but it's burried under a mound of papers with similar scribblings of random numbers *** eachother every which way, making finding it a lil' time consuming... lol (it's prety simple math any way, if you know anything about dmg ratings and pDif values it should be easy)