Why would you even buy it again at all then? Either you have played it before and don't want to play it again or you will pay for a upscaled/improved version. Your only issue seems to be price.
rp2000
i would buy it for online.
the online completely sucked on ps3, lost my character several times, etc.
there were zero missions to do just tedious repetitive stuff.
they made survival crap as well as making it impossible to make money without cheating/glitching.
the online was turd, hopefully by the time a PS4 version arrives they have fixed online, but then I would only want to pay £20 max to play it online, as i'm not interested in the story mode at all, i've been there, done that and got the t-shirt.
in fact i don't know anyone who hasn't played it, even people who have never gamed in their lives were talking about how much they loved it, i mean even women i know who saw their kids playing it and liked the look of it, peoples 50-60 year old dad's, etc. people who weren't even huge gamers and their consoles had sat there collecting dust for years were talking about it, etc, etc.
i sold my ps3 copy to a female colleague after i bought my ps4, it's amazing the diversity of people which were playing it and talking about it.