If you have a wireless network at home (eg, if you have ADSL and a wireless router) to connect to the internet, and you don't feel the need to be able to connect to the net from anywhere else, then the Wi-Fi one should be ok, because you'll connect to the internet through your home network.
With 3G, the device itself will connect straight to the network the same way a cell phone does, so you can connect anywhere, but you'll have to pay for that bandwidth (and as above, you'll need a sim card). Out of bundle 3G rates tend to be about R2 per MB, so it can get expensive if you're not careful. The in-bundle rates are much better if you get a 3G contract or buy a data package from your provider (cell phone company), but unused bandwidth tends to expire at the end of the month.
If you do want to be able to get the thing online, and you don't have wireless internet at home, or you want the option of being able to connect wherever you are, then you should go with the 3G version. Assuming the 3G one has Wi-Fi as well, you can use your home Wi-Fi connection when you're at home, and 3G when there isn't a Wi-Fi network available.