Over the past years, RTC has become closer and closer to the original games (and thus, CSBWin). When I came to this forum in early 2005, I didn't like RTC; sleeping took forever, monsters attacked you immediately they came on the adjacent tile to yours etc. Even though there still are differences (projectiles fly faster, monster can split up and regroup, spellcasting monsters and DES VEN are way overpowered), they became smaller and smaller.
Looking at the custom dungeons that got released, I'd say RTC is definitely more popular for designers. The reasons are more or less obvious: you have less limitations in size, number of monsters per level etc., it is easier to implement your own graphics and sounds, and getting into some advanced stuff is easier in RTC (though you can probably do more with DSAs once you get into them). These are exactly the reasons why I switched my dungeon from CSBWin to RTC.
As for playing, I still prefer CSB. The real gem, however, is probably DSB. It plays faithfully to the original games and has monster movement sounds. Plus, it is certainly the most powerful engine when it comes to building your own stuff. We only need a really awesome dungeon to prove it (something like, oh I don't know, Dark Portal 2 maybe
).