Aside from the obvious...this is the Touring forum so it's not tough to guess what we like, you'll be VERY happy with the King over the Softy...I also looked back in '07 because they do look cool...but after one ride, the decision was very very easy...actually, just sitting on the King vs the ST it was clear...but I am 6'2" 240 and the ST just felt cramped.
I was considering the same two bikes last fall. Rode both the king and the heritage, and to my surprise, I liked the king more. Then the dealer said "you gotta try the street glide". I did and was immediately hooked, needless to say, I bought the street glide. My advice - ride em all.
I agree you need to ride both. Myself, and this is only my opinion, I had a lowrider 2004. Loved it. Took 2200 mile long trips. Then got an 07 fatboy. Fixed it up, 103ci, rode two up on 2500 mile trips a few times, no problem, love it. Now I have a 2010 roadking. My friends have Ultras, great bikes but I like the style of the king. They are the same bike from the seat down. I dont listen to radios when I ride so the king was for me. I am only 5'8" so put low profile shocks on. Set them at 28, ride two up, love it. My point is I have loved every bike I have owned. Each has it's own personality. You will love whatever you choose. Ride on.
I had an 06 Heritage until last summer when I traded for a SG. The Heritage was a great bike, low center of gravity nice fit and a comfortable seat and you can ride all day on it. I would rate the SG (RK) a better overall bike but not by much, I enjoyed every mile on my Heritage.