Quite honestly, I don't see much point in 3G at all. Since I have my Kindle I haven't had the Wifi on once. Oh yeah, once to try it out, then it went off again.
It really depends on your usage. If you want to subscribe to newspapers and other small pieces of text appearing frequently such as blogs then maybe 3G makes sense, but if you only read books, then how often does it happen that you finish a book on the road - unforeseen - and desperately need internet to buy a new one? Get some free books as spares onto your Kindle in that case
Just my point of view. I could even do without the Wifi if I needed to.