I have the d90 with the twin lens kit (18-55 and 55-200).
I also have used the 18-105 and must say it is 1 cut above the twin lens kit.
Nothing specific in handling or usage, however it just appears to be overall that little bit sharper.
Whatever is decided, I would recommend buying body only and picking 1 good lens to start with
My brother has the 5100 and got the twin lens kit it comes with (55-300?) his long zoom certainly appears to be better quality than my 55-200, as I believe they are a newer lens to the market.