I've got a D3100 and love it. I only have a few bad notes:
1. it has absolutly terrible high ISO noise, I mean really bad, so low-light capabilities are pretty much nil
2. it has a few too many not needed buttons
I cannot vouch for/against a D5100, but I know of a new D5200 that you may want to look at, but frankly I think it is a step back for low-light with Nikon now cramming a bigillion pixels on a crop sensor. If you have the cash, I haven't heard anything bad about a D7000. Definitely invest in glass, though. I bought a 18-55, 55-300 (soon to be replaced with a 70-300 and maybe a 50-500 Bigma) and a 16-85, which I absolutely love. Built like a tank, and IQ
is fantastic. I would get a D3100 and buy the 16-85 or 18-200 for medium tele, 10-24 for airshow wide, and 70-300 for tele work. Glass will last forever if treated right, bodies need to be (or you will want to ahev them) replaced about every 5 years, so invest in lenses.