A 7mm Mag won't gain much velocity with a barrel over 24", but it certainly won't start to lose any either. If it were in a bolt gun I'd say 24" is all you really need unless you are planning on pushing the round to the fringes of it's capabilities.
Up to a point barrel length is more about how the gun looks and balances. On the TC Encore, the receiver will be about 3-4" shorter than a bolt gun, so even a 28" barrel will have about the same overall length as a 24" barrel would on a bolt gun.
Get the one that you think looks best to you and suits your hunting style. The 26" barrel will be shorter overall and handier in thick stuff without sacraficing velocity.
Barrrel length does effect velocity, but 2" either way is not enough to ever notice any difference in performance in the field. The length has almost zero effect on accuracy with scoped rifles. If anything the shorter barrel would be more accurate in theory. Stiffer barrels are usually more accurate. Everything else being equal, a shorter barrel is stiffer than a longer barrel.