What about CANON EOS REBEL T1i Kit - NEW w/18-55mm Lens
That is a step below the level of a D90. The D90 was hurting T1i sales so badly Canon had to introduce the T2i, and even that didn't help a whole lot.
For shooting sports and break dancing you need to be looking at auto focus modules and frames-per-second (fps) rates.
The D300 has the Multi-CAM3500DX AF module with 51 focus points, 15 of them cross-type points, up to 7 fps.
The D90 has the Multi-CAM1000 AF module with 11 focus points, 1 of them a cross-type point, up to 4.5 fps.
All Nikon dSLR camera's have color-aware metering, Only 3 of Canon's higher end cameras have a color aware metering capability.
The D300 has a magnesium alloy body and weather sealing.
The D90 has a plastic body and no weather sealing.
The D300 I have for sale was used almost exclusively to shoot sports.