I personally don't think you'd ever know the MTS has 12HP less than the Diavel. Both bikes are scary fast. I think the MTS is a more versatile bike overall, plus it has more bells and whistles standard. It is probably more suited to some off road stuff too, like gravel roads, etc. The MTS look is something you either like or don't...personally, I've always liked the look. It looks like some kind of Raptor bird...like a Hawk. There's a few things to be mindful of with the MTS: The seat height is pretty tall, so if you're a short person, that's something to be aware of. It's not heavy, so it's not a HUGE deal, but takes some getting used to when stopping at lights, etc. I had to sort of cock my ass off to one side of the seat in order to get a good purchase with my foot of choice (usually the left). I understand they have improved the seats a lot, so an aftermarket seat might not be necessary now...the older models had seats that were like sitting on a 2X10. I put a Corbin on my '05 and solved that problem. Also, because I have a 32" sleeve length, I put risers on the bars to get them up a bit more...again, I understand they have improved the bar position now. You have to be really aware of how fast you are going...the bike LOVES to run, and you'll be going much faster than you realize, really quickly. I looked down one day on a long stretch of straight road (with line of sight for a good mile) and was doing 110mph and still had another gear left. And that was on the old air cooled motor...the new one would be MUCH quicker. The problem, if you want to call it a problem, is that the bike is so rock solid and handles so well, the perception of speed changes as you grow accustomed to the ride. The suspension and brakes are so good that it actually improves your riding ability...in the twisties, there's nothing like it and I never came close to reaching the limits of lean angle, and I consider myself to be a pretty aggressive rider in the curves. The accessories are kind of pricey...the full Terg exhaust system will set you back about 2K. But that includes the ECM modification, K&N filter, and the exhaust system. There are some other systems out there too, for a bit less money. And you can just put slip on mufflers to get a bit more sound, with the stock header pipes.
The Diavel Strada has smaller bags and lacks a few bells and whistles. I have tried to go on the Ducati site and see if it can be fitted with some of the things that come on the MTS, particularly a luggage rack or topcase, but the site is super slow today, or my connection speed is down. The Diavel has a lower seat height and more upright sitting position, with the bars coming back a bit more, especially on the Strada model.
I would not hesitate traveling on either bike, but probably would not be doing many 500-600 mile days on either. But, I'm old and stiff, so the leg/feet positioning would start telling on me after 300 miles or so. But a 300 mile day is, to me, a nice, reasonable distance to travel in one day, particularly if you stop and smell the roses a bit. With either bike, you'll need a GOOD Full face helmet...something like the Schuberth C3 would likely be a good choice. Not having external speakers on a bike is not a big deal for me...even on my CVO bikes, I usually listened to the stereo through the headset in my helmet. So using an Ipod or something similar and streaming via Bluetooth would work fine for me.
Both bikes are engineering marvels, and if you like something "different", excellent bikes. But they are 180 degrees different than a HD. Personally, I have probably owned my last HD bike. The Ducati forums are also different...some good technical information, but don't expect a lot of great social interaction like there is here. But this site is unique in so many ways, not even other HD sites compare, IMO.