As far as I understand it, a Surface and Surface Pro run completely different types of software. Windows RT cannot run normal Windows Apps. I would view this as a major qualitative difference in the type of machine it is and such a similar naming strategy is confusing and not warranted.
I think they're both actually looking like pretty good machines yet again and both have their uses, but I think the message about what they are is going to be confusing to the marketplace because of the naming. I don't work in branding for a big corporation, so maybe my opinions are wrong. But Microsoft ended up with a warehouse full of unsold Surface RT's last time and I think that this is a big part of the reason why.
The Air/Pro/normal macbooks all run the same software binaries. Microsoft was/is selling two products with different machine architectures that run different software but with almost identical names. That's a marketing and customer-service disaster.