Hmm, choosing a singleplayer RPG over an MMO one.
Although I had my fair share of gripes with Dark Souls in the past, the PC port disappointing me aside, it actually won me over and I pushed through the game and finished it after almost 100 hours of gameplay. Since PC will be the lead platform according to them, I want to give them another chance to see if they manage to pull through so I will most likely grab that game for my PC.
ESO on the other hand, from the footage I've seen from Quakecon, I can honestly say that I was majorly underwhelmed. The animations both in 3rd person and first person were more jenky than Skyrim's one. Add that the fact that they decided to go subscription based alongside buying the box is kinda crazy in my eyes. Way too much to pay so I can have a WoW like quest system and more people in my game. I'd rather go play Skyrim more instead.
So it's kinda up to you OP. If you enjoyed Dark Souls you should go for that.
If you like the Elder Scrolls universe you might want to check that out at least for the initial 30days free time when you purchase the game. It should be more than enough to max out your character and decide then at the end of the month if it has any endgame if you should continue subscribing.