I have used both. I started with Turbo (center, circle shaped) one that I used for 5 years then I bought a second set that was the Accelerator (offset, oval shaped). I didn't know the second set was the Accelerator until I got it home and started using it.
Being used to the Turbo I immediately noticed a difference. The A pedal literally accelerates your stroke making more of a snap of a stroke so your sound is a little louder initially. Once you get used to the feel of it you could play at light volumes easily. The T pedal is a smooth as silk motion that just does exactly what you do with your foot. Soft, hard, slow, fast, what your foot does the beater does.
After playing with both of them I kept the newer A pedal on my kit as I was playing rock stuff at the time. If I had a lower volume gig I used the T pedal though I could easily use the A pedal.
A cool thing on the new 9000 pedals is that they can set the cam at either setting or in between. I currently have mine set at the full Accelerator setting.
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Jeff Porcaro - Lord of Groove