To me this seems like a win for you either way you decide to go.
On the front end of that three year window you mentioned I would give a slight edge to Letang based on two factors. One being the team around him the other is the fact that he has logged more ice time at the NHL level. That's where I think Letang gets the edge over EJ over the next year or so.
The fact that he's logged almost a full season more worth of games is an invaluable leg up here. He's had more time to adapt to the game on the big stage, which temporarily trumps any edge EJ might have over him in skills. For what it's worth I think that's where EJ's slow start has come from. The competition has had chances to see his game, make adjustments to it, and now it's back on him to make adjustments to his game. As we get closer to the back end of that 3 year window, the scales may start to tip more in EJ's favor.
Yes they both are getting some pressure from behind from Goli and Pietrangelo, but I think they both can hold on to their spots. That said even if they get passed by those two are still no worse than the #2 man in the top D-man pairings for their teams. There's still plenty of value there even if that were to happen. Personally I feel confident EJ and Letang will be their teams #1 option on the blueline going forward.
Like I said I think this is a win/win no matter which way you decide to go.
Your beer mug isn't half-empty, your just half-way to the next refill.....