I lean towards more capacity VS connectivity.
The way my family uses the iPad2, it was smarter to forgo 3G and buy high capacity.
On the off chance we do travel with it, we always have our iPhones to fall back on.
With more memory, we can fit lots of media to minimize the need for cellular data.
Too much space is better than not enough.
Buy highest GB capacity that fits your budget.
As far as the added expense for 3G, that should be incurred only if it's known for sure that it will be used.