I suspect the 'frustration' is that he has bought a fancy camera, that has serendipitousely delivered a shallow focus effect, that micro-sensor compacts and camera-phones, exploiting the facet of incredibly short focal lengths to reduce the critical-focus range, and draw the near focus distance so close to the camera,so they are 'focus free', cannot achieve. Hence buying an 'enthusiast' crop-sensor camera, curiously mismatched to the longer lens for a larger 35mm frame camera, that exaggerates that shallow-focus effect, has delivered it 'by lucky-accident' and resulted in pictures that to a non enthusiast audience display a 'quality' they associate with 'professional' photo's, encouraging the OP to engage in what is more usually a 'professional' discipline, they do not have the expertise know how for, and to continue with the misguided pre-conception, re-enforced by the apparent confirmation that their 'professional' achievements are the result of buying professional gear, so to continue the trend and make more professional images as exemplified by shallow focus effects from fast apertures, they need even more professional gear and still faster aperture lenses,, not professional know-how.... beyond what pro gear a to buy!
MY fear here is that so ill equipped in know-how to even be asking this question, how many disappointed couples and families there may be or disputes and arguments, when the OP's results don't meet their expectations, from their lack of camera know how, and reliance on equipment....
I think Phil once commented that the job of a wedding photographer is 99% cat herding in chaos and 1% pointing a camera! If they are good at the cat herding bit, perhaps not knowing too much about cameras may not be such a problem, but otherwise? NOT an arena I have EVER wished to engage in!!
I think that the most useful, but probably least welcome advice would be to learn about selective focus techniques, lenses, and cropping effects, learn an AWFUL lot more of the actual craft side of the job, as well as the business around it, rather than diving in all credit cards blazing looking for a gadget to do the job for you! But still... just my opinion.
To answer to the original question; whats the advantage and disadvantages of Full-Frame to Crop?
Crop-sensor cameras are more common, as are lenses and accessories for them, hence usually they are cheaper, and more available. Other than that? Pretty much comes down to actual and more specific requirements and actual and more specific camera comparisons, where its not which is 'better' or 'worse' but where the overall and inherent 'compromises' are least for the job.... which is back to top and 'savvy' over 'gadgets'... as my old wood-work teacher used to say, 'power-tools are a wonderful thing to let a craftsman make more, more easily, and fools to more and more easily eff-up! If you cant make an accurate hole with a hand drill, you wont make one any better with a Hack-and-Wrecker!, its the crafty that makes the craftsman, not the tool-kit!"
