All three models are pretty close in features. The 3500 has a modulation wheel and a floppy disk drive. The 3000 does not have a disk drive and a modulation button in place of the wheel. The 3100 is almost exactly the same as the 3000, but the 3100 has a mic input jack. Aside from that they all have the same sounds, recording functions, synth features, styles, ect.
GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.