No matter where you go for fantasy football insight this year, you're likely to find that the rankings and projections you're provided have rumors have crept up about him not seeing his full compliment of touches in the Seattle offense as youngster Christine Michael begins to take over.
But even if Lynch doesn't give up a good amount of his job to Michael, he's still not as efficient of a runner as Forte, and won't be nearly as involved in his team's passing game. Lynch is a strong pick in the first round, especially in standard scoring leagues, but nothing about his profile causes him to stand out ahead of Forte, apart from the potential of scoring a couple more touchdowns than the Bears' runner.
So whether you're in a PPR league or a standard scoring league, if you're left with a decision between Forte and the field with the fourth or fifth pick, Forte is the right choice to make.