For me, the reason I'm going to be buying both is just simply, I am a fanboy of the series and am very grateful it's releasing on PC.
Reasons I'm buying it on PS4 and PC even though I own the PS3 version -
1. I have my PS4 setup in the bedroom, So I'll be able to easily play it whenever I like "Likely online"
2. The PS3 is in the lounge room and my partner likes to watch her shows, So It'll be easier to get it on the PS4
3. Sometimes I like to be half sociable and hang out with the family / her in the lounge, So buying the PC version for my Metabox Gaming laptop -
a. Helps rockstar out, and shows them we support (The PC Edition)
b. I really really support mods
c. I was hoping I could cross play PC/PS4 so I could continue my online experience on the laptop and then return to the PS4 version at a later date
d. Allows me to drain the boredom of Home and Away by playing this.
Now all I need to do is think of which one I'd be playing online more with... If I end up playing online on the PC then there is no point in me getting it on the PS4 really.. So I was thinking of
1. Get the PS4 version for playing / online
2. Use the PC version to Mod / Sandbox around when bored in the lounge
I guess there's nothing stopping me from creating 2 characters and playing 1 on the PC and one on the PS4, But that's annoying, it's hard enough to level up and get money the legit way on it now IMO since I don't play it a great deal.