What is the real differnce in the 500 vs the 590SP?
Brent pretty much answered all the questions, but I'd like to add a few things for your consideration.
Converting a 500 to a 590 actually costs more than the 590.
My friend bought a 500, but wanted to convert it into a 590.
This was his total cost:
-Mossberg 500 out the door: $340
-forend tube (to make it possible to install a shorter forend): $80
-forend nut (to secure the forend tube): $20
-Knoxx stock: $50
-Mako forend: $50
-Finding out he can't install a magazine extension: priceless
If you are determined and have the money to buy the 590, by all means, buy the 590. Do not buy a 500 and then try to convert it into a 590, it's not worth it.
Hope that helps