Eastside Guy, I can't comment on the Nait5, as I know nothing about it. Can only talk to the 5i, 5x scenario.
I find the more my 5x breaks-in, the more I like it, finding it very smooth indeed. And I concur with Emil that there is no upgrade path for the 5i. If you do your homework you'll see that there isn't much in the way of published spec's on the 5i, there is a lot more going on under the hood of the 5x, not just the upgradability.