Hey Dan,
Welcome! The Road King would be my personal preferrence keeping in mind it would be in addition to my hot rod FXDX. Here is a little elementary tech that you may or may not already be savy to.
The Road king has a rubber mounted Twin Cam 88 engine (Alpha Motor), and a frame very similar to the old FXR. It is of the FLH or touring family, where as the Heritage is of the Softail family with a counterbalanced, solid mounted Twin Cam engine (Beta Motor) More cruiser oriented with a nostalgic flair.
The TC 88 B has alot more moving parts as opposed to the TC 88 A what with the counterbalance shafts and supporting chain drive system. This imparts a nominal power loss and a lower RPM rev limit. The suspension travel of a Softail is somewhat limited to preserve the form over function hardtailesque styling.
A friend of mine and service manager at my dealership has a '95 Bad Boy(Softail) and can't take the beating it gives his lower back. After Three hours in the saddle, he can hardly walk. The Road King suspension is of a much more compliant touring oriented design. Much more comfortable for the long haul.
In my opinion, the Softail is a pretty poor copy of the old hardtail design. There are more realistic and functional designs out there such as Atlas, Daytek, Ness, Rubberglide(monoshock, under seat mounted). The list goes on.
Don't get me wrong, the Softail continues to be Harleys most popular model and I wouldn't mind owning one. The aftermarket has just improved upon their design in some cases.
The Softail has been improved upon by the factory also what with the B Motor and totaly redesigned more rigid chassis. The Choice is ultimately up to you and your intended riding style. Personal taste and preferrence play a major role in our choices.
Good luck and happy shopping!
Mike