Since you already have an iPod touch, I would recommend the Sony 350. The 950 is really, really great, but I find the really convenient thing about the wifi is on the fly email, wikipedia, etc., which you already have with the iPod. If you're frequently away from a computer it could be useful to be able to buy books over wifi/3g, but many people don't end up buying books that way all that much. The in-store features of the nook are nice, but unless you already spend a whole lot of time in B&N you're unlikely to use them much. At least that's what my boyfriend found with his nook. (B&N has been a bit stingy with the cafe coupons recently, too, that's what really got us in.)
If you're having a really hard time deciding, I would recommend getting the 350 from a store with a good return policy and using it for a week. See if you miss the wifi. If you do, return it and try the nook or the 950. Or get the nook to try and if you find yourself in B&N reading it all the time, that's probably the one for you.
You will probably be happy with whatever you end up with, they're all good devices and it seems like any one of them would be a decent fit for you.