What should I buy?

Sleeping Dogs all day. Fighting, racing, pretty cool (if stereotypical) story, and, most importantly, some stylish outfits.

Why not both?
Insert 'both gif' from road to el dorado here

Saints Row IV isn't a game - it's mostly a joke, fit only for those who enjoy stupid humor. Saints Row 2 was awesome, but after that, chaos ensued.
Sleeping Dogs is an awesome game. You can rarely find games with such a beautiful storyline and so detailed melee combat moves.

Never played Sleeping Dogs but I did play SR IV and it's really fun, just don't go in expecting the usual GTA-like sandbox game, it is a sandbox game but more akin to superhero games. It doesn't feel that much as a sequel to SR3 gameplay wise, more like a spin-off; even tho it uses the same map and graphics and it's a direct sequel as far as the story goes.

sleeping dogs for sure, amazing game, a shame the studio closed down

http://store.steampowered.com/app/8870/BioShock_Infinite/
http://store.steampowered.com/app/8930/Sid_Meiers_Civilization_V/
http://store.steampowered.com/app/213670/South_Park_The_Stick_of_Truth/
http://store.steampowered.com/app/255710/Cities_Skylines/
http://store.steampowered.com/app/201810/Wolfenstein_The_New_Order/
http://store.steampowered.com/app/260230/Valiant_Hearts_The_Great_War__Soldats_Inconnus__Mmoires_de_la_Grande_Guerre/
http://store.steampowered.com/app/241930/Middleearth_Shadow_of_Mordor/
http://store.steampowered.com/app/110800/LA_Noire/
http://store.steampowered.com/app/242050/Assassins_Creed_IV_Black_Flag/

I know your poll is irrelevant, but I must say I enjoyed both games equally for what they were. Sleeping dogs is a much more hard core experience, mixing the combat of the Arkham games with a more 'gritty' crime story. Saints row is much more like an over the top action movie. Dumb but surprisingly sincere and committed to being ridiculous. It may have started as a GTA clone but morphed into something unique, a self-aware joke in many ways, that fully embraces the weird side.
And incidentally I prefer both of these games to GTA. Something about the tone of the GTA games just doesn't sit right with me. Supposedly 'mature' and gritty, but most of the 'highlights' seem to involve college boy level sex jokes. SR does over the top ridiculousness better and sleeping dogs (and other games) do combat and action better. Anyway, that's my unpopular opinion.