There is no such thing as a 4G iPhone.
The iPhone 4 would be the better option for some things as it is obviously faster and has more system memory. If proximity sensor issues are not a problem for you that is, amongst the myriad of other issues.
The 3GS, if you can get that, would be a better buy if you don't care about cutting edge.
If it is a true 3G (which sounds iffy), I would pass. 3GS or iPhone 4 only are worth it. The 3G is already obsolete in terms of applications that can use them. Even the 3GS is being left behind for some apps.