I had both actually. Used the Straggler for about 3 months or so, years back now so all I remember is the rear dropouts being a nightmare (though people insist that using better qr levers solves that).
Have more experience with the Kepler (disc). Felt the same as Surly, heavy, dead, slow and boring (But that's just me buying the wrong bike really, neither of them made a good winter road bike. Went for a Tripster instead and not looked back since).
From memory Kepler wasn't as long as the Surly, Brothers were also doing a discount on carbon fork upgrades (perhaps they still do). Not sure if the new Kepler is flat mount, but thru axle alone makes it better than the Straggler I think.