Prof at small university or postdoc at big university Recently I've been faced with the dilemma of being an assistant professor in a small rural University near my home city or being a postdoc at the top University in my field in the world.....

What are the advantages or disadvantages of either choice?
As others remarked, this is highly individual. To make the answer easier, i'm going to assume that you don't have a strong personal preference and there are no factors that would 'make the decision for you', such as your partner working near one of those places.

Being a prof at a local university would likely mean a safe, stable position, so that you can focus on living your life. It might also be fun, if you can find motivated students or the curriculum offers something unique. On the downside, funding options might be very limited in such universities. You could end up in the vicious cycle where grant applications fail because you don't have enough research output, but you don't have research output because you have no money to fund a proper group. This of course depends if the position is a teaching or a research prof.

Smaller universities also sometimes attract people who, pardon my French, 'didn't make it'. If you're unlucky, being surrounded by such individuals can bring you down with them. You can imagine where that would go. 

To be honest, i am facing a similar decision now, and am opting for the postdoc option. The reason is simple: freedom. Especially in a good university, you'll meet plenty of people that can steer you to a more advantageous situation, and the extra experience will only help. It also gives you the time to build a wider portfolio, and hence apply for better prof jobs. You will likely be less pressured to look for funding and have less administrative work, giving you the time to focus on your ideas, supervise students, build your own course and so on.

Two-three years from now, when you'll be finishing the postdoc, the local uni will perhaps still be available as an option. If you go for the prof position, switching jobs will be much more difficult. You do the math.