D5100 or D7000?

You ask about a good macro lens for $300-$400. This might be another plus for the D7k over the D5100. The D7k has an AI coupling ring allowing it to meter aperture priority and manually with older excellent manual focus Nikkors.

I recently picked up a 55mm f3.5 Nikkor AI Micro for $37 including tax. If the lens was any sharper I'd be shaving with it. I'm having just too much fun with it.

The lens mates and meters perfectly with the D7k, with the D5100 not so much (does mount however). Now a 105mm AI Micro might be better for working distances and you can still find them well below your price point.

When you consider being able to use screw focus lenses and MF ones (absolute bargains), it quickly makes up for the price difference between the D5100 and the D7k. Then you get the added features of the D7k and the choice is a no brainer.

If all you're interested is some video with an articulating screen and a 18-55mm, 55-200mm or other similar SWM combo then go for the D5100. Otherwise, Go for the D7000!