How long is this 'long distance'? If you want a good estimate, search for 'iron butt association' and get a rough idea. You will get a few good laughs as well.
Ducks are good bikes, but maintenance wise, they cost a bit more in terms of time and money due to their complexity. HD is all over the US so if your long distances will cross state borders, assuming you are in the US, then HD is the better choice in terms of dealer support. Beemers are similar to HD, just not as wide. So if we are to go by dealership support in case of gear related issues while you are on the road, I recommend HD.
In terms of the bike itself, if comfort is what you want, then HD is the better choice. Just so you know, I do not -- DO NOT -- own any HD bikes. But I do ride an HD derivative: Buell S1 Lightning. Buell went out of business in 2009. Buells are not considered HD by the HD crowd but are considered HD by the other crowds. So am not saying HD because I am an HD fanatic like so many HD riders/owners are. HDs may not be technologically sophisticated as Ducs or BMWs but unless you are delving into the guts of the engine and transmission, you can do most the maintenance and repairs yourself in a weekend afternoon. If you will take a passenger, then the Dyna is definitely better for that than the others.
So...If you are taking a passenger, and predictably luggage for at least a weekend, then you really do not want anything below 1000cc. Assume you are a manly macho man of trim 100 kilos and your hot woman of typical supermodel physical attributes, then you are looking at combine weight of nearly 200 kilos on the bike, not counting the mass of the bike itself. This kind of load prefers something with a lot of low end torque with minimum throttle movements to get the mass initially moving. Large displacement V-twins are known to handle this easily.
Another consideration for the 'sports touring' category is the Kawasaki 1200R. I wish am a little bit taller to ride this bike but I know a lot of people who have taken this bike over 1000 miles long rides -- repeatedly. This bike is in the same class as the HD. The in-line 4 liquid cooled motor by Kawasaki is as near as 'bulletproof' as engineering can make it. This is not a P-51 Mustang but a brute: P-47 Thunderbolt or F-4U Corsair.
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