Yeah, for me, this isn't a matter of choosing Young Arlong or Halloween 1, the correct answer is both.
Gotta use those gems. The Halloween fortnights just offer too much value, you're getting the shot at socketing 4 Strawhats with each run, not to mention all the skill books you can get.
I think HW2 is even better: Zoro is absolutely worth skill-maxing, I think Brook will be solid if I can get his skill within usable range, and I haven't had a chance to socket any of my Robins yet. The Sanjis will all go to Candy Sanji, who I've found to be surprisingly useful.
As far as clearing HW1, I remember using SWS Zoro the first time it came around to tank a hit from Franky so I could build up some specials to beat Luffy. Besides that, I think Nami and Chopper are actually the dangerous parts of the fortnight: you need to be able to 2-shot Nami so she won't blind you, and Chopper can be hard to kill after he buffs his defense (and he does a lot of damage). As long as you can avoid taking hits from both Chopper and Franky, as well as one-shot Luffy, it should be easy. I'm just using Mihawk/Mihawk, Arlong, Dock Lucci, Mr. 1, and Law, going for pure power.