I think the difference is pretty simple.
In "normal pc may become a" supernode mode, Gnucleus decides if your connection is fast enough and if it makes sense to become a supernode.
This setting is good for most novice users, because they don't know a thing about supernodes.

Dedicated supernode is an option for expert users who want their servant to act as a supernode all of the time, no matter how much bandwith there is available.

__________________
sandman

Artificial Intelligence is no match for natural stupidity. -Unknown