Eve is a sandbox game and its one of the best if not the best out there in my opinion. I would play it above all other games if I had the spare cash for p2p games.
Price of sub $15
Awsome Graphics Card $150
Your first time getting a PvP kill in your new battleship. Priceless.
Free to download but pay 2 play. Though if you have the money to subscribe to it then its well worth it, but the game does have a steep learning curve.
I just started playing Wurm Online and Eve funnily enough. I'd say from early indications that Eve will have more longitivity, however Wurm Online is exceptionally good too. You can just start building stuff wherever you want basically. Think Minecraft but in the real world and you have Wurm Online. Heavy emphasis on community projects and the community from what i've seen so far is the best from any game.