The big one is the SA voices! Body designed changed..., impoved sound quality, and quite a few other things. This issue did come up when the T2 was released and the discussions were whether the T1-T2 upgrade was worth it.., and if memory serves me correctly many found the upgrade to be worth the money.
Also..., wasn't the original T1's mega voice use reserved primarily for styles and not real-time play? I think that was an issue as well. If you can get the T2 and it's within your budget I'd suggest buying it. I was pretty impressed with the unit every time I demoed it.
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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.