Letang consistently produces elite offensive numbers from the back end, and is honestly probably a little bit underrated defensively. He can be a useful all situations defenceman, and log a ton of minutes.
Chabot doesn't have quite the same productivity, and really isn't anything to write home about defensively. He still has at least as many lapses in his game as Letang. I guess he's a bit "bigger" than Letang, but in the way they play...i'm not sure that actually means much. If anything, Letang is probably the more physically engaged player anyway.
Can't really think of a reason to take Chabot over Letang for right now. The real factor in Chabot's favour, is his age if we're talking about picking between them for now and for the future. But in this case, we're not. So...i take Letang.