Dark Souls 2 isn't on sale, Dark Souls 1 is. And yes, it's definitely worth $6, but only if you have a controller, and if you're willing to put a little work into modding it. Just know that if you go into the game with the wrong mindset, you will get incredibly frustrated and probably quit. It's not an easy game, but for the most part it's a very fair game, and every time you die should be a learning experience. If you can't get into that then you'll probably be flushing $6 done the drain because you won't finish it.
As far as borderlands 2 is concerned, it's a great game if you have someone to coop with all the way through it. It's a lot of fun to play with friends, but only if there's no level disparity and it's a lot better if all of you are going through it together for the first time. I'd say that playing it alone isn't worth it, and this is from someone who has beaten it on every difficulty multiple times and has multiple max level characters.
But yes, both games are definitely worth owning and playing. Whether you'll enjoy them as much as I have will probably depend on what mindset you go into each one with.