Really, it's too new to tell since it's only been out a short time. All I can do is speculate on theoretical longevity. I have used the 840 Pro drives, they're great. TLC flash is a newcomer to the SSD market but it should last. I haven't used the 840 Evo drives yet, so, I have no first hand experience with it. Between the two, you will probably not notice any difference as they are comparable in performance. Everything I have read about MLC vs TLC is based entirely on speculation as far as which will outlast the other. It seems TLC could have more of a caveat towards failure rates since it uses more bits per cell but again its speculative since the technology is so new. I would wager they both should last the same amount of time with a lean towards TLC since it is a newer technology. It's kind of like when manufacturers add new areal density to new hard drives.
EDIT: If you are really concerned about longevity, you may want to consider the 840 Pro as it has a longer warranty.