I've ridden and ride sportbikes and Harleys. I've test ridden the 848 and 1098 and chose the 1098, but was used to a Beemer with 160 crank hp.
Biggest thing, as mentioned earlier for you will be comfort. A sportbike is far more capable of cornering, accelerating and braking. On the whole, it is far less comfortable than a cruiser for slow speed riding. Depends on the Harley, a hardtail chopper isn't exactly a comfy couch and handles really, really bad. A model like a RoadKing is like a '56 Caddilac and is good to carry shit, passenger and cruise around at 55 mph seeing the country side.
Coming off something like that, either bike is going to be like suddenly switching from a '56 Cadillac to a dedicated track car like a Porsche GT3. You will be gaining a lot of capability and dropping some comfort and riding them in a straight line at low speeds aren't what they are designed for. You may want to keep your Harley and get the Duc Superbike as a second bike and finances may push you more toward an 848 then.
On the road, coming from a Harley, the change to either will be so much a change up to performance and handling and a decrease in comfort and passenger comfort. The 848 will have more than enough power for street use. 600 cc jap sportbikes have more than enough power for street use. You can also spend a lot of tracktime and not master a 600cc or 848cc for awhile. Although I own a 1098, there are plenty of 250cc riders who could kick my butt on a racetrack, so it is not the size of the dog in the fight but fight in the dog.
All that being said, why did I get a 1098 rather than an 848 then? Well, because I could. I cut my 848 test ride short to spend more time on the 1098. It didn't feel like too much power. It felt just right.