Hydreigon has a base of 600, while Sigilyph (not sure on spelling either) has only 490
Hydreigon can use moves such as Fly, Ice Fang, and Fire Blast to cover its weaknesses between bug, other dragon types, and fighting
Sigilyph can use limited coverage moves, Like Smack Down and Shadow Ball. They have decent power, but they could be wasted because of Sigilyph's okay attack
I would say in a battle between these to, Hydreigon would come out on top
But training to Level 64 is a risk you have to take
Hope this helps! :)