I actually have both, but since I feel I've already seen enough shirtless Cu Chulainn for a while, I've mostly focused on prototype Cu.
and honestly he's pretty good, he's reliable like normal cu thanks to his 'protection from arrows' active ability but he's more offensively focused compared to normal cu.
He lacks the guts and stance renewal ability but instead has the ability to increase his critical damage and damage against beast type enemies. I also have a good amount of luck with gae bolg, it's honestly a 50/50 rate with the OHK ability on a normal charge.
not really
since the 'protection from arrows ability' blocks 3 attacks, not just all attacks on a single turn, I find that it often lasts for two turns at least. with some skill improvement a lot of times the skill will recharge before he's even run out of evasions.
you should bring at least one tank character though, like sieg, along with you so you have someone to eat the shots for cu. in most of the event quests Cu is a solid member of my non-berserker team alongside sieg.

How is Proto Cu as a Servant, I gotta wonder?