As long as Shonn Greene keeps fumbling I'm sold on LT in the short-term. He may be fading a bit as this season goes on, but Forte has been outright terrible all but 2 or 3 weeks this year.
So it really comes down to how much you care about the long-term ramifications: In a keeper, Forte is still the better guy to have even if you're buying LT over Forte for the rest of this year, as I am. LT might be done after this year. Even if he's not, he doesn't have that much more left. There's some risk with Forte that he'll lose the starting job to a better back if he keeps playing poorly, but generally it's clear he's got far more upside as quickly as 1 or 2 years out in this deal.
I'd lean toward Forte for that reason, but as to your specific question about for the rest of this year only: It's LT.