Advantages of higher quality cranksets
They will definitely be lighter, and probably shift a little better. More rigid is possible but not especially likely, as the cheap one will be strong enough to take a lot of abuse (they don't particularly care about weight so that's easy to do). 

(edit) A cheap crankset very likely has the chainrings rivetted on, and they're not designed to be changed. Sometimes the smallest chainring is bolted on and can be changed. Swapping one of these for a more expensive one that has interchangeable chainrings will probably give better shifting.

Cheap bikes are the ones more likely to be bought by people who either weigh a lot or are careless with their bikes, so are more likely to be overloaded. More expensive bikes get crashed instead. The cranks are designed accordingly.

As discussed in  Suggestions for progressive upgradation of bike you're almost certainly better off saving up for a more expensive bike rather than trying to upgrade the one you have. Buying parts retail is a lot more expensive than getting them as part of a bike (manufacturers buy 1000's at a time and don't pay for retail packaging).