I think Amazon has the best store. Barnes and Noble's store doesn't work well with an iPad anyway, I push buttons and nothing happens all the time. Kind of buggy. Also, I don't think their customer service is quite as good as Amazon's.
Oh I see you're in the UK. Well B&N isn't available there anyway.
I don't know what's there other than Amazon, but it is a really good store with an amazing selection and great customer service if you have a problem.