Both are good. And you don't need to get real close to the insect... If your tokina 100mm f2.8 is a macro lens, it means that at its MFT, it will be 1:1 on your sensor, if you need to get closer to the insect, you can either,
1) Add a Raynox or other brand's close-up filter.
2) Add the extension tube/ tubes
3) Use a reverse lens
4) Use a reverse lens on a ordinarily mounted lens.
If you need higher magnification (more than 1:1) try using a old nikkor 20mm f/4 AI or 20mm f/3.5 AI lens reverse mounted on a PK13 extension tube, it gives a magnification of about 2X. Cheap & sharp combo. I use this when shooting insects.
Alternatively you can use close up filters from Raynox, Hoya, Nikon & Canon to increase the magnification of standard micro lens if you need > 1X magnification.
The D5100 with a crop factor of 1.5X will have the benefit additional magnification because of its smaller sensor.
If you're using the same lens, you can get equally close with either cameras.
But you have more pixel density on the D5100 (photographing the subject with more pixels) and because of the crop factor it will be apparently closer on the D5100 but actually the same as D700.
Same lens so sharpness should be same on the lens side of the equation but I haven't seen comparisons of acuity between the D700 and D5100 sensors. They have different AA filters, microlenses and different default sharpening so its not an easy thing to compare.
But all else being equal, shooting at base ISO with plenty of illumination you should get a more detailed picture with the D5100.