Well if you think you will definately use the 3g/4g then the answers obvious, if your never going to be putting any films or large numbers of pics then the 16gb will be fine, it's surprising how many apps and stuff you can get on 16gb... (well it's actually nearer 14gb.)
I'm retired and we tend to have a lot of holidays 8 or 9 a year with quite a few short breaks inbetween so I do like to have quite a few films installed, infact I have about 40 of my fav films on mine & still have about 15gb free, though it is the 64gb version... was hoping they would have increased it to 128gb ;-)
I was very undecided myself yesterday whether to buy Just the Wifi version at £559 here in the UK or to pay £659 for the 4G, and was still undecided until the very last minute. I have used the 3g a few times on my iPad 1 when on holidays etc, but figured as I have the ability on my smart phone it was now less important on the iPad. In the end I went with the 4G version and payed the extra £100. I suppose mainly because I'm fortunate in that the £100 was not a great amount to me, and I suppose even if I may not use it now, I may do so in the future, so I suppose I opted for it just in case I needed it at some time, as obviously it's something you can't add later.
In the USA I believe 4G is getting more widespread, while here in the UK it hasn't even been ratified properly yet and IMO probably won't be widely available across the UK for another 2 or 3 years at least not mainstream anyway, if we had got 4G now I don't think my indecisiveness would have been so bad...lol
So.. go for the 16gb if you definately will use the 3g/4g and your sure you wont be wanting to put any films ect on it.. enjoy.