I am a dentist, fwiw, and own a D700 and D300, but have had Canon digital gear over the last 8 yrs also starting with a D30 to the current XSi, which I still own also. The advantages to FF are specific, some like wide angles are now wide angle lenses, based on 35mm think. FF I feel gives better tonality, and re: D700, significant drop in image noise at higher ISOs. My default for walking around daytime is 800, and I could go higher and still make large prints without fault. That is about it, my D300 images look at least as detailed, have better sharpness at the edges with current lenses I have. I refuse to buy and use only MF primes to achieve this, but do have some, like a 40mm CV and 28/2 Nikor, but for general imagery, I like zooms and use them. My favorite kit for the D300 is a 16-85VR and 70-300VR but even the 55-200VR is incredibly sharp.
Now one can buy better glass and achieve the same thing in FF, but with two big negatives for me, first the weight and second expense. I find this hard to justify, and will wait for the next camera generation to arise and decide on which system I want to expand further. Bottomline, when printed it can be extremely hard to tell differences, I print large 14x21 and bigger, up to 24x36. The differences are small between two 12mp cameras, and often the DOF adventages to DX overway the smooth tonal transitions of FX. I guess I could easily live with either system, but can have both. I would love a MF dig system but I must be too poor a dentist to afford that, or maybe use the word JUSTIFY such an expenditure with two kids in college. I also don't know at 50+ whether I want to carry such loads anylonger. I used to shoot in MF and LF, but today is anew day....hope this helps...if not just ask. Don