The 580 has centerboards which will kick up and pivot into the hull if grounded - no harm done bar a bit of cosmetic.
the I20 has daggerboards which do not kick up if grounded. the good news is that the daggerboard will break rather than the hull. saying that, i have grounded pretty hard on a 20, eased the sails, wrestled the daggers up and continued many times.
The 580 looks a good boat but suffers from not having a racing fleet to fit into. It reminds me of the N6.0 but with centerboards.
The 20 is certainly better suited to a spinny, but the 580 may fit the bill for you.
Using a spinny is not as dramatic as you may think if you pick your conditions and know a few basic principles that the guys on this forum are sure to impart....