The sole difference between 16GB and 32GB is more storage space, and you have to have some idea of your usage patterns to determine how much you'll need - will you store a lot of files, pictures, videos and music, run tons of apps.
The 3G question is more about accessibility - if you have HotSpot access points where you are most of the time, skip it. If you're using it while mobile - walking, in a car - or in remote areas, 3G can be handy.
If you don't know any of this, my motto is "when in doubt, max it out." Get the 3G with the most storage space you can afford, to cover all the bases. If you discover you need 3G later, you're screwed. If you discover you need more space down the line, there are workarounds...