Well if you call 2:1 PS2 like then I guess you're right. I just don't think 100m vs 50m~ is the same as 155m vs 24m.
PS4 is tracking way ahead of PS2. I don't think it'll get to 155m, but they'd need to fuck up badly to not go over 100m sold.
I can't see the XB1 selling 50m at all. There's only real interest for it in the USA and that's significantly muted compared to the interest that was in the 360. And before you say "but the 360 was a slow starter", Xbox is now an established brand. It should be flying out of the gates like the PS4 or the Wii.