If you have to between those two: PS4.
But I would go with PC. Why? Last generation I would have supported Sony and I know this is a bit nitpicking but I'm really mad that I can't play my PSN (at least that bros) on my PS4. I got like 40+ PSN games and that is terrible.
Honestly now-a-days I play 95% of my games on PC. Hard to go back to consoles and their limitations.
PSN PS3 games were still built for the CELL processor, just like retail games. If Sony including backwards compatibility for PS3 games the price of the PS4 would be like $550 right now, because they would of needed to include PS3 hardware in the PS4 box, not to mention the PS4 would be one monster of a box!
It's a choice they made early on to leave out PS3 BC, a smart choice no doubt. XBOX One is the same way, it can't play old XBOX 360 games because they are a different architecture then x86.
but it's not all bad, i know like with Flower, Sony made a PS4 version, and if you already own the PS3 or Vita version you already own the PS4 version. of course that's not all PS3 PSN games, but it shows they aren't trying to rip you off and makes you pay for things you already own again.
Theres a rumor floating around that native local based PS1/PS2 emulated BC is coming in a patch for PS4 in the future:
http://www.neogaf.com/forum/showpost.php?p=105962159&postcount=778