I personally prefer DarkRP. If the server is well done you can have very nice Rleplay scenes. Also, there is a lot of diversity on Gmod like Metros 2033, Stalker etc... After that, GTA RP is still a good game but it doesn't dethrone Gmod
I agree, with the new S&BOX aka Gmod2 coming out I think it'll still be on top in terms of customizability, accessibility, and modding capabilities. it's just great and i don't see gta 5 rp coming close to that. However I have seen lots of GTA 5 rp servers blowing up lately so for all i know that's the new thing
DarkRP is just sitting in your base for hours as money printers tick and getting killed by a pay2win person and GTA RP is weird, a lot of stuff to go through and most of the people who play it have a stick up there ass.