In my expirence, I've found the mini's to be higher producers than the Nigerians.....unless your buying Nigerians from some of the more well known, succesful dairy breeders in the country.
In my area, to buy a good Nigerian doe from a breeder who shows, health test & breeds for production & ease of milking your looking at about $500. In contrast, my miniature Alpine has a good udder, is a FF, milks an average of 6lbs a day (slightly more than 1/2 gallon) & I bought her bred (so I didn't have to mess with a buck) for $250. Yes, I can get Nigerians in the $200 & below price range, but those are from the folks who breed for flashy colors/blue eyes for the pet market & I would only buy a Nigerian from someone who has a reputation for breeding dairy animals.....Anything else is a waste of my time.
I've seen some impressive 8lb a day mini lamanchas, & for mini's I really like the first generation crosses.
On the other hand, had I invested in the Nigerians I looked at, I would be able to sell those kids for more than I can sell miniature kids....at least in my area.
Since you need two goats, get one of each
I started with mini's because I thought the standards would be too big for my kids (& besides the price I thought the Nigerians may be too small)..... Now I'm phasing out my mini's in favor of standard Alpines & Lamanchas....turns out the big girls are easy on my kiddos, & I like the longer legs giving me more room for a milk bucket