By better game, what aspects we talking about? Gameplay? Cause I couldn't stand playing GTA 4/RDR/MP 3 with the clunky player movement, cover system, or restrictive mission design but haven't played GTA 5. Or the open world is just better with side quests or activities? Which I can believe more.
They're two of my top 5 last gen games, but I've played GTA V (not including online) over 500 hours but SD barely 100 hours. Biggest difference is scope.
Sleeping Dogs is basically designed as a great crime movie in terms of pacing, world, and scope. GTA V is much more like a drama, and I think much more enjoyable if you play it slowly a little every day over many weeks like watching a drama, taking the time to really appreciate and engross yourself in all the small details of the world and grow attached to the characters. The story is better that way. SD, on the other hand, is paced fast enough that it works with movie pacing. The gameplay and world design lends itself also much better to a faster playthrough, where as GTA V gameplay gets better with age (if you play on manual aim) and most of the world, weapons, cars, etc go unappreciated unless you spend hundreds of hours in the game world. I've beat GTA V almost 6 times now, and I'm still finding new surprising corners and details in the game world, fun new places to have cop or gang shootouts, enjoy experimenting with different weapons or cars, and in general just free roaming with different ideas every day. SD doesn't have the same scope to do that. But it has a perfect simplicity in its storytelling that is cliche but still manages to make you care about the main character yet retaining a great movie-esque pacing speed.
Plus the character Winston is a spitting image of a friend of mine, tattoos, hair, chain, tanktop and sweatpants, and all. So the story is a little bit charming in its cliche self-awareness.