The short answer is that all else being equal FX will give sharper images, at least in the centre of the frame (possibly not in the extreme corners with wide lenses). The long answer is here.

The typical argument that gets rolled out against full frame is that lens sharpness falls off toward the corners, so using the same lens on a crop body avoids the softest parts of the image circle. This is wrong on at least three and a half counts.

1. Firstly centre sharpness will be 50% higher when using exactly the same central portion of the lens on a bigger sensor. A lens resolves a certain number of line pairs per millimetre on the film plane. When comparing images you should do so at the same resolution/output size. With a 16mm tall sensor a lens resolving 200 lp/mm in the centre will give an image with resolution of 3200 line pairs per picture height. With a 24mm full frame sensor the same 200 lp/mm translates to 4800 lp/ph giving you more resolution hence greater sharpness.

2. Not all lenses are softer in the corners, this applies mostly to wide angle lenses. By the time you get to 85mm you can expect excellent sharpness across 90% of the frame or more, so with a little cropping you can get better resolution than a smaller sensor even at the borders.

3. With a full frame sensor you can stop down whilst still achieving the same depth of field as a crop sensor (as to maintain subject size you have to get closer, which reduces depth of field). Stopping down often results sharper picture until you hit diffraction effects.

4. Finally centre sharpness often has a much more dominant effect on perception of sharpness as the subject of the photograph is often at or near the centre of the image, and it is very rarely in the extreme corners.

---

Whilst that technically answers your question, I would not advise anyone to upgrade to full frame to fix the problem you are experiencing.

You are really not comparing like for like. The stock image you posted looks like a studio shot, with powerful studio lights allowing a very small aperture to be used. It has also been extensively retouched. That is totally different to a handheld outdoor shot you seem to have taken wide open at f/2.8

Compared to upgrading you'll gain far more in terms of sharpness by:

• improving technique (eliminating subject/camera motion, meticulous focussing)
• paying attention to lighting (certain lighting conditions enhance textures)
• finding the sharpest aperture (usually between f/5.6 and f/11, depends on the lens, experiment!)
• post processing (there are some advanced techniques, deconvolution, octave sharpening)

Finally, whilst it is good to strive for the best result in camera, I'm afraid you will never get the sharpest possible result without postprocessing - if you shoot RAW (which you ought to do, for stock). When shooting JPEG a lot depends on the camera's sharpness setting (which is incidentally akin to sharpening in post) so beware of this when comparing results from other people's cameras.