I went to the dealership and sat on a 1300. I'm a relatively big guy at 6', well over 200 lbs and the 1300 was uncomfortable for me. I was a little too close to the tank... it just didn't feel right. Then I sat on the 1800. I was home. I love that bike and ride it every chance I get. I came from an Aero 1100 and it just didn't have as much get-up-and-go as I wanted, especially getting on the freeway with a passenger. With the 1800, I can't even tell I have a passenger. Great bike. I'm sure the 1300 is a good bike too and has the advantage of having a single-pin crank, so it has that potato-potato sound that we all love, but I just like the feel of the 1800 and I absolutely love the performance.
My 2 cents.
definately have to disagree with the size issue. i'm 6'4" 275lbs with a 36" inseam and my 1300 still feels huge. i grew up riding my dad's road kings and my 1300 is quite a bit bigger than that..
my wife and i do plenty of 2 up, she's a little tall too at about 5'8" but she doesn't add much weight at 125lbs.
but with our combined 400 lbs of body mass and a loaded luggage rack and saddle bags we'll ride for 4-5 days and 1500-2000 miles with no complaints other than the seats, still averaging better than 40 mpg, better than 45 if i'm easy on the throttle.
the single pin crank does make a huge difference in the sound, i love the sound of my 1300..and i also have to add that i believe it to be a major pro to be able to rejet and tune my carburetor for MUCH less than a 350 dollar power commander for the efi system on an 1800.
the extra horsepower of a 110" motor over an 80" motor sure would be nice while i'm out with my buddies, but for the added costs that come with the 1800 (aquisition, fuel, engine tuning) i would get a 1300 again in a heartbeat.