i don't know a whole bit about the advantages. BUT, i do know that i don't change the line on my spinning rods as much as my spincast.
However, with my spincast i can be a hell of a lot more acurate casting!!!
About the different companies, the best thing to do if you want to make a purchase go to a fishing shop with a decent selection and ask them if you can just try them out.
Look for the smoothness in reeling, the balance of the reel(does it pull to one side or not), how much line it can hold and what the weights are.
Basically, if you are looking to fish for a certain species, buy what you think would be best. OR if you are looking for something just to mess around with to see if you really want to get back into it, buy a nice medium action that will hold all smaller fish and most big.
also, when buying a spinning, look at how many ball bearings it has. Although you just want to try it, it is important. Well, to me anyway.
I can't afford to buy the reels with seven bearing so i usually go with three or four and they work great.
you'll realize the difference when you're fishing believe it or not!!!