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.
The original Surface lineup was very compelling. The only real issue with it was the majorly confusing marketing message with having two products with a major qualitative difference sharing essentially the same name.
It kind of pains me that rather than learning from their $900 million mistake, they've essentially doubled down on it and somehow managed to create two products with even more similar sounding names. I think that they're once again going to confuse some people about the respective capabilities of each machine with this naming scheme.