I've typed on my dad's Galaxy SII before and I really suck badly at it, so I'm thinking about going with a slider phone.
Obviously, the SIII has better hardware (1.5Ghz instead of 1GHz, 2GB RAM instead of 1GB, 16GB flash vs. 8GB, 720p screen resolution vs. 480p, 1080p video vs. 720p, slower...