I have a D70, D300, and D810. Shooting lossless RAW, using a ColorChecker to develop a unique profile for each camera, untwisting each profile with dcptool, and then white balancing off a ColorChecker in each lighting condition, each successive camera is an improvement over the previous one. The step from the D70 to the D300 is significant but the step from the D300 to the D810 is absolutely staggering.
That's a roundabout way of saying that I have the same cameras and my D70 is spanked heartily by my D300 in a fair fight. Better dynamic range, better color accuracy, better color gamut all go to the D300. Obviously, better resolving of fine detail goes to the D300 as well. You are probably just better at working from the starting point the D70 gives. Profiling both your cameras properly, including using dcptool to untwist the profiles, and some practice would almost certainly yield huge improvements in the look of your D300 results.