I am one of the few musicians here that uses mostly all MIDI. These are my views based on that. There are plus and minuses to both MIDI and RTs/RDs.

If I were you I would for now use RTs and RDs. They will give you instant gratification. The plus side of these is they sound great right out of the box so to speak.

The minus side is you get what they give you. You can cut and paste sections together and in RealBand (which I do not use) you can regenerate areas of your choice. However in most cases you can not get them to play exactly what you might want them to play.

The plus side of MIDI is that you can get them to play exactly what you want them to play. You can add parts, change sounds and notes very easily.(Personally I find this easier than cutting and pasteing RTs/RDs but YMMV).

The minus side of MIDI is that it costs money to get great sounds and you have to learn MIDI to get convincing parts. As you mentioned MIDI sounds from computer motherboards a terrible, but MIDI sounds from more expensive VSTis like Kontakt are quite good. But, and this is a big but, you have to learn how to control MIDI via various MIDI messages (CCs, mod wheel, pitch bend etc. Don't worry its really not hard.) If you buy a good MIDI sound source and drop a BiaB file into it and expect it to sound like a RT good luck. For piano, organ and drums it's great but for wind and string instruments not so great. This is where those messages come in.

So I would start with RTs/RDs. If you want more control I would start with a good MIDI drum sound source for my MIDI drums. Drop a BiaB MIDI drum track into the VSTi and modify it so it fits what you want to do. After you master that do the same with piano and organ and continue to the wind instrument and strings.

I will try to drop a good example of what MIDI can do in the Off-Topic forum a little later.

Again this is just my ideas and it may or may not be what you are looking for.

Good luck from one guitarist to another.

Whenever I get something stuck in the back of my throat, I dislodge it by drinking a beer.
It's called the Heineken Maneuver.

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware