First, sadly, there is no 16GB RAM option for the 13" Air. If there is, I cannot find it on the Apple Store site.
As to i5 or i7, I have the 2011 11" Air with the i7 and I had the 2011 with the i5 before that (for about a week.) The reason I upgraded was because I wanted/needed the 256GB SSD and the only model in store, at the time, with the 256 was the maxed out i7 model.
Honestly, I did not notice a real difference between the i5 and the i7, performance-wise, with Windows (I run 7 64bit in Parallels) Of course this could be a RAM limitation since the 2011 models maxed out at 4 GB.
However, sadly, I did notice a definite battery time difference. The i7 gets a lot less time on a charge. (At least an hour.) Hopefully the new Haswell's do not suffer from this. Just in case, though, I ordered the new model with an i5. Battery time is key to me.