Hm I really dunno. I don’t like Isaac but for entirely personal reasons. You may like it. So I’ll leave it to the other Chronies to say their bit, since I’ve got no way of helping you there.
Plus, the sale goes until the 5th, so take a deep breath and let’s wait to see if anyone will come over to help you with your choice! (: 
@Agzer if you played Binding of Isaac (OG) and liked it but wanted to re-experience the progression with a few new tricks, go for it.
If (as is the case from steam library peeking) you haven’t tried Binding of Isaac, I would say buy the Wrath of the Lamb edition (which is unfortunately not on sale right now, but routinely went on sale for quite cheap!). And see if you like it.
And I can’t say much about the Long Dark, haven’t really been following it that hard.
I feel like I’ll spend a ton of hours on BoI, is the rebirth one worth it? Or should I just get the OG Plus wrath of lamb? I now have £12 since I refunded battleblock theatre so I can buy the long dark and the og binding of Isaac 
I am unfortunately not the most adept at answering this question… having played BoI on and off for two years or so… I have a friend who really loved the original, and enjoyed the added challenges that rebirth added. However, I am uncertain how much of the enjoyment of rebirth was due to having lots of prior experience in BoI.
For myself, after investing 80 hours (online) plus who knows how much offline into the BoI+Wrath of the lamb, I am unlikely to want to switch over to rebirth…
I believe this might be some money laundry scheme between Gnuffi alts…
From what I understand rebirth is basically a remake of the original but with a different art style, more stuff in it and the interactions between the items are more interesting as they synergise better. I am no expert but I would go with rebirth, its newer, better and there has been content content still coming out for it, even recently. Either way to unlock everything there is in this game will take you a long time, so be prepared.