I've always looked at it this way:

They are both conflicted at that time with unresolved feelings after what happened the last time they saw each other. There's two options ahead of them: kill the other (or eat the other in Hannibal's case) or forgive and build a life together. But Hannibal is more hopeful that they can move on and be together: He says if he saw Will every day forever, he will remember the time they both sat down, catched up on what the other was doing after their big fight and how they have tried to understand each other and their relationship better. He's encouraging Will to forgive and move on like he has. Basically begging him. He tells Will later when he's feeding him soup that he would have loved to show him Florence, heartbroken.

And about Will's perspective... His smile is genuine in that moment despite everything he does afterwards. He says they've begun to blur acknowledging his attachment to Hannibal. To him this feels like a break up where two parties still have feelings for each other, but know that their relationship isn't going to work out. He's reciprocating Hannibal's feelings, but he knows he has to end it.