I would like to thank you all for your replies.
At this point I feel the duty to summarize my learnings from your posts:
For macro photography, there are two meaningful ways to compare crop vs. full frame sensors:
The comparison can be made trying to achieve the same image in terms of perspective and depth of field.
* In order to get the same image in terms of perspective (related to distance) and framing, the smaller sensor shall be coupled with a shorter lens. If we put a 100mm lens on the full frame, something in the range of 60mm lens should be mounted on the crop sensor camera (assuming a 1.6 crop factor).
* In order to get the same depth of field, the aperture on the full frame camera shall be roughly one stop smaller. The smaller aperture would loose one stop of light, that would be compensated by the "one stop higher" light gathering capacity of full frame sensors.
* Identical framing is achieved with the same distance to the subject.
==> In such first comparison the output from the two setup is theoretically very similar. Anyhow, in practice, as pointed out by Neuro and proved by Mackguyver, the image from the full frame sensor would be visibly sharper. Since there is a proof, I would say this conclusion is incontrovertible.
The comparison can be made keeping the same lens on both full frame and crop camera.
In this case the distance to the subject can be either be selected to be the same (2.1), or we can move the crop camera further away (2.2) to get some sort of "same composition" (will never be the same composition because perspective would be different)
2.1) keeping the same distance:
The crop sensor would capture exactly the same image that would be obtained from the full frame sensor cropping in post production.
==> The advantage of crop camera would be in pixel count, allowing more latitude for further cropping (if ISO is not pushed up too much) - as Neuro was writing elsewhere, the subject image is captured with more pixels.
==> The advantage of the full frame would be a larger angle of view, which is potentially important for multiple reasons (e.g. easiness of composition)
2.2) moving the crop camera further away from the subject.
Such scenario would could be considered less important; a full frame owner could move his camera further away as well, then crop in post production, and get the same image that could be obtained by an APS-C camera owner (except from pixel count).
In this latter scenario (2.1 and 2.2), the crop camera have actually the advantage of capturing the same cropped image that a full frame camera can capture, but with more pixels (usually it is the case with current cameras, may not be true with future cameras). This is unquestionable an advantage, if the ISO is "low". It is not and advantage, and can be a disadvantage, if the ISO needs to be pushed up.
Besides the above, there are some advantages of current crop cameras that I am not considering, e.g. swivel screen, weight, and some sort of better autofocus (7d coupled with 100mm f2.8L IS USM).