Would need to know how many you start at each, scoring (is it ppr?), etc. I'd keep Calvin over Jacobs, especially since you already have Forte, unless you have some unusual scoring or starting requirements.
In a small league, it is really important to get a stud player and hold onto them. Everyone will have very good players, so it is much harder to win with balance because everyone will have balance. In a 12 team league RBs are such a precious commodity you have to stockpile starters. With only 8 teams and 16man rosters, there will be RBs available throughout the year. Especially with Jacobs having a limited shelf life due to his age + running style, I think Johnson is an easy pick here. You might try to analyze who others will keep to convince yourself there'll be a decent RB left at your draft pick. I think if you do that you'll see guys like LT or Westbrook or one of the rooks will be available and you won't care as much about keeping Jacobs.
Calvin is my #1 keeper/dynasty WR, so I would keep him as my second keeper. I do like Jacobs, but they do share carries a lot in NY and Jacobs does seem to get banged up every year. Calvin will be a stud for years to come.
In an 8 team league...everyone is going to be stacked so if you have the opportunity have a top player at a position like Calvin, you keep him. One of the few times you can afford to keep that WR over a solid HB is in situations like this in a smaller league.
If Detroit turns the corner, you will probably not get another chance to have Calvin Johnson on your roster unless it is in a blockbuster trade. This guy is scary good -- even with ho-hum QB's like Orlovsky, Kitna and Culpepper. If Stafford works out, the sky's the limit for Calvin Johnson.
Jacobs just got his big-money deal and could go the way of Shaun Alexander and Larry Johnson, who fizzled after big bucks kicked in.
Stick with Calvin Johnson. There aren't many WR's I'd recommend over a starting RB, but he's one of them. He could be a fixture on your team for the next decade...
I agree with everyone else, Calvin is simply a beast. If Stafford works out, and they are surrounding him with talent then he should produce, so Calvin should keep putting up big numbers for the next 10 years. I like Jacobs, but there are 10 other rb's that are just like him and you can get, there are only 2 or 3 other wr's that are in same boat as Calvin. Good luck.