If you travel (within your own country) a lot then surely 3G would be helpful so you can go online when there is no WiFi? That said, if you have an iPhone anyway then you could tether it to the iPad using the iPhone as a mobile hotspot and therefore removing the need for 3G; this is presuming your network/carrier allows this. If travelling outside of your country then 3G would not be so useful as it will likely be cheaper to find and pay for a WiFi connection.
Next questions is memory size - are you going to ram it full of films to watch while you travel, therefore needing something like a 32GB or 64GB one, or is it just for a bit of music and Internet browsing etc. meaning 16GB should be fine?