Well, even if it doubles the price, i think that getting Carrara 5 Pro for let's say 25€ is really worth it. But it's true you can't say its really free anymore !
Edit : To answer your last question yo can have a look at the C6Pro feature list and remove the what's new section of C6Pro and you'll have a good idea of what's in it.
Cheers,
Laurent aka Tartiflette