I'm not sure what the difference is, I started playing with the GOTY and I was surprised the DLC was included in the disc itself and it wasn't a redeem code. What I GUESS it might be is when you have the GOTY edition, the free versions will be available
for you to use, but like I said, it's only a guess
The free one is on there so that you can play with others who have the DLC. So if you don't want the DLC, but still want to be able to play with all possible players in co-op mode this enabled that feature.
The premium version is specifically there for users who want to journey to those places on their own, and earn the achievements.
As regards DLC should be free. I suppose you would also think premium parts for older cars should be free as well right?
Other than what everyone else has said the free versions of both do one other thing, if you get them and go talk to the NPCs which will be located on the river under the bridge in Bowerstone then they will each give you a unique item.
I believe with "See the Future" you would talk to the NPC "Murgo the Trader" and he gives you a Jester tattoo and i believe a pair of steam-punk esque shades.
With "Knothole Island" the NPC is parallel to Murgo and should sound slightly familiar if you played Fable I, and he gives you a pair of Plate boots and a Roman style helmet.