Neptune = 3.4/3.5TH and 2K at the wall per Tolip_wen
SP30 (Not looking at a late October batch) = ~4.5TH and 3k at the wall.
2x Neptune = 7TH and 4k at the wall
2x SP30 = ~9TH and 6k at the wall.
Realistically for equal power to the wall you can run 10.5TH/s with Neptunes.
Right now the availability on both are close to the same (or you can buy someone elses pre-order).
Why not add the prices too? Neptune was sold for over 3$/Gh while the SP30 was sold for less than 1$/Gh. The B3 Neptune is still selling for 1.7$/Gh so I wouldn't call the availability close to the same.
With the money spent on a B1 Neptune I could buy 2.5 SP30s for a whooping 11.25Th/s. Ok even if I can't split the SP30 in 2 I would still get 9Th/s and still have some money left in my pocket than getting a 3.5Th/s burning monster.
I find that the only reasonable competition to SP30 would be ANTMINER S3 -B5 and it is shipped on 7th August.