It depends, really. When chatting with somebody, writing stuff or organizing various sites like Google Calendar, Backloggery or Goodreads, PC is preferable for me - nothing beats keyboard+mouse when it comes to fast and precise input. However, when I'm watching YouTube, reading articles or just surfing around, my 10'' Android tablet is more convenient for that - I can just lie on my couch and swipe things
It also depends on my mood and condition - when I'm coming home from a brutal shift, I'm usually too busy/lazy to boot my PC up, and since my tablet is always "ready to go" (unless it's uncharged or something), I just use it instantly; however, on weekends, it's more common for me to use PC, since I'm already doing all the organizing stuff anyway.
If there is a choice of "What to buy exclusively for web surfing", I would suggest a tablet, though. It won't offer a perfect experience (again, you'll probably want to buy a separate BT keyboard, and there are some known problems with stuff like SWF files), but finding a good and cheap tablet is much easier task than finding a good and cheap laptop (you can find a decent tablet for as little as $65-70 on eBay - passable laptops usually cost twice as much), and there are tons of specialized app versions of various sites on both dominating mobile OS's to make things faster and more comfortable to use compared to browser versions - Windows 10 is trying to do the whole "app" thing on PCs too, but those attempts are rather pathetic compared to what we have on Android and iOS.