Out of those two, I'd say definitely Black.
Start as new as possible if you are new to Nuzlockes. I'd recommend X/Y as it's a simpler game, the EXP share makes levelling from scratch much less stressful, and there are plenty of freebie Pokémon given in or near cities (if you treat cities and routes as one encounter each, this is a big help). Gen 5 and beyond, TMs are reusable. Before that, there's a risk of teaching TM moves to Pokémon that may die. Plus you can take advantage of all those Audino - they throw the fight and give you crazy good EXP. The only tricky bit is battling N and Ghetsis back to back depending on your interpretation of the rules.
Then again, this is all about adding as much challenge as you can handle, so that may sway you towards Diamond. There are all kinds of additional restrictions you can add to the classic Nuzlocke that can make any game borderline impossible, but you may also value your social life, so it's up to you to find the right balance.
