They both go 75 mph. The Road King loves to go 75 mph...all day. The Sporty loves to go 75 mph too, but if you have an old butt, all day on the Sportyis a butt burner. Your mileage will vary.
At 100 mph, my RK is silky smooth. At 100 mph, my old 88 Sporty would vibrate my feet off the pegs. The Sporty will get to 100 mph quicker, but you won't want to spend as much time there.
When I rode a sporty... at100 mph I looked like Don King, and felt for the most part I was hanging on for dear life!!! Touring bike at 100mph is just a cruise in the park!!! jmo
There's not enough difference in the two to argue about when it comes to 0-60, 1/4 mile or top speed. By today'smotorcycle standards, ain't neither one of them quick or fast.
I have a 07 roadking and have been on my friends 07 sportster. The sportster is faster but the roadking feels better at high speeds. I also have a 1977 sportster, I would not want to go over 75 mph. with that bike.