I have Nigies and love them, but I have heard that the crosses (Nigie x Nubian or whatever) often tend to get the best of both breeds. So don't forget to look into those as well.
I have both and what it comes down to for me is $'s.... Nigis cost about $175/head/yr to keep and produce on average 3-4lbs/day..
Nubians cost about $600/head/yr to keep and average 6-8lbs/day...
For me the cost/lbs of milk Nigerians are the way to go since it cost 3x as much to keep a Nubian and you only get 2x the milk. And you can get more freshenings out of the Nigerians since they can be bred year round and Nubians cannot.
If I was getting a dairy goat, I'd get a Nubian. I just LOVE the ears, and I'm enjoying my wether!
Nigis are nice too though; they're sweet little goaties.