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I think themeparks are WORLDS better for legitimate pvp than sandboxes.
Eve for example.
The game is based around this skill queue where you can just pile on skills and they'll train and you'll get whatever benefits when they are done training. The problem is that you can have any skill you want, no skill cap, so players who having been playing since the game released have a distinct advantage over any players that haven't. I know eve doesn't have traditional combat, barely combat at all. But what it comes down to is your ship and the books you've trained. Not actual player skill. And other sandboxes that have come out are similar to that, AoW with inner/ scrolls, DARKFALL with the redic grind(I'm not sure about the new one never played).
Themeparks to me just seem a whole lot more sportier, instead of the whole "you haven't played for a year so I shit on you instantly" deal. Devs usually work aggressively on the class balance(not counting non triple a mmos), you can't stack to many things on one character for the most part, op items are usually banned from pvp, the list really goes on and on.
TL;DR: Sandbox = get shit on by people who have played since day 1, or just generally play a lot more than you, then strive to grind endlessly till the day you MIGHT catch up with them.
themepark = play a little bit, get geared quickly, become competitive then strive to become a better player to beat BETTER players