Hi Mike,
Welcome. That's a lot of questions.
Firstly lets lay some ground work. DCC is DCC, RailPro is RailPro. They are different and not compatible. You cannot control DCC with RailPro and you cannot control RailPro with DCC.
There is no easy way to compare what you might "miss out on", you'll miss out on certain aspects from the other no matter which way you decide to go; it pretty much boils down to which gives you the feature set that you most desire. Probably the features that most come to RailPro for come under ease of use (no CV programming, no speed matching, simple MU'ing of locomotives, no remembering locomotive addresses). The only feature that I can think of that can be given any serious consideration for going with DCC is your locomotives can be controlled by another DCC system.
As to the best sound, well that's objective of course. I've never heard TCS's sound but I have heard SoundTraxx, QSI and Loksound and from my experience Loksound can't be beat. TCS (and Soundtraxx, QSI, Loksound) make equipment for DCC so it can't be controlled by RailPro so sorry, no TCS WOW sound if you go with RailPro. That means yes, at this point in time you are "tied" to the sounds from Ring Engineering. Some sounds you can create and load yourself, horn's and such but prime movers not yet. Hopefully one day.
LocoNet is a Digitrax thing, and it's the way commands get communicated between the throttle, command station and other DCC devices on the layout via a cable on a DigiTrax system. Other DCC systems have the same type of thing with different names. You don't need to worry about it with RailPro as RailPro is direct radio control vis no cable.
JMRI is an open source software project with a few different components. I'd say the most used component is DecoderPro, which gives DCC a bit of a more user friendly user interface for working with and programming decoders. You don't need to worry about this with RailPro either.
Signalling, Your guess is as good as mine without reading what those people were saying. I do know that DigiTrax sells detectors that detect where a train is by sensing the current draw (I think) but how it all ties into their system I do not know. You can do train detection with RailPro too.
Route control, that's simply using DCC decoders to control turnouts. You can write a program (probably similar to a macro) that changes a whole bunch of them at once setting a whole route in one go. RailPro could probably do so just the same but it doesn't yet. I wonder if anyones even asked Ring for this feature?
To wrap up, for the size of your proposed layout, RailPro would suite you quite well and I believe that you'd get a lot from it. If you were to choose DCC, then for that size layout with just one operator I'd actually recommend an NCE powercab. DigiTrax has a of hype simply because it was the first US made system on the scene. There are a lot of users and clubs that use NCE too.
- Tim