what difference does it make if i download the sims 3 rather than buying it?

No crash or lag, as long as your computer is reliable . Plus you save money because if you were to buy these expensive expansion packs/stuff packs you would be spending 100+ $ on it . No need if you can download it free with instructions from youtube . !!

If you use a cd your spending less time worrying about if it will work or not, because you might have to download the nocd mod ( easy installion via youtube ) NRASS website . Search it .

So your game can tell you bought it and wont open up and message saying it cant verify it has been purchased or not .

Torrenting makes close to literally no difference between normal games except in most normal games that you torrent you will not be able to play online (with a few certain and specific suggestions). I suggest that you just torrent it but make sure you get some sort of peer blocking program such as Peer Guardian or BTGuard so that in the event that your IP is traced by local officials or somebody similar, you won't be tracked down properly. If you do not wish to take these risks, just buy it.

There's no difference but a downloaded copy (pirated one i suppose) won't allow you get the latest patch for the game. This is also illegal.

The difference is that one is legal and smiled upon by gamers alike (as it allows companies to provide us with more products and services). One is frowned upon and those who commit it should be hung by their genitalia and pecked by crows.