Which is the better form of government: a unitary system or a federal system?

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Depends from case to case. Some countries are best fit for unitary systems, some for federal. I don't believe in one size fits all. 

I couldn't see a some large counties, such as U.S. or India, or medium ones, like Germany, functioning under unitary.

I couldn't see such country like Poland functioning under federal either.

[MASHED POTATOES. VOTE!]

Federal is of course the only solution for large countries or states composed of many nations, ethnicities, linguistic groups or races. Unitary systems are largely inefficient and outdated. Devolution of any kind is always preferable.

Of course there are small countries with federal system. Switzerland for example. Yet, Swiss have a long tradition of federalism. I wouldn't see any purpose of converting a not-large country with unitary history (and pretty unified society), like Poland into federal one.