I have never owned none of the SPs but I think first u should look at what the 505 and 808 has to offer. Basically compare them. Below is a link to where u can see all three of them being compared. Hope that helps.
The 505 is more user friendly. I make my beats with that but the 808 has the built in 4 track which coudl help alot. You should check out the 808EX its a nice looking machine.