I am a complete amateur, but can give you a little bit of advice when it comes to the nikkor lenses. I have an APS-C nikon (D5200) camera and when I just got the camera, I had the opportunity to test out the current version of the nikkor 105 mm lens for a week or so. The lens is absolutely awesome, but I in my opinion you really need a full sensor body for that lens to be usable for anything other than real close closeups of flowers or plant parts, anything else and the working distance to get say a 6" diameter plant in the frame of the camera becomes quite large. I realize the lens is not meant as a general use lens, but having it function as a true 105 mm lens on full sensor body gives you some options if you do not constantly want to switch lenses, which using that lens on an APS-C body does not.
I now have a micro-nikkor 60mm lens with my APS-C sensor body, which still gets me the real close closeup shots at a quality that I would say is very close to the 105mm micro nikkor (I am certainly not a good enough photographer to see the difference in shots that I have taken with both lenses), but I can also get away using the lens to get less closeup images without having to step back too far from the subject, or constantly feel that I should switch to a different lens.
I also tested out an older 35mm manual nikon lens and I must say that I came away very satisfied with the experience. The autofocus lenses allow me to be so point and shoot that I do not always take the proper time to properly set up my shot, whereas the manual focus made me consider things a little more because I was bringing things into focus manually. I know I can turn my autofocus lenses to manual, but so far I have not done so... too lazy, I guess...
Regarding the mega pixels: I think I would generally tend to agree with William as to the 10 - 20 megapixel difference, except when it comes to blowing up and cropping closeup macro shots even further... a RAW 20 mpix image definitely gives you more leeway to play with that, but those files do fill up your hard drive quickly...
