My quick compare of Circuit OG and Circuit Tracks

Circuit OG | Circuit Tracks
----------------------+--------------------
Mini-Midi 2x | Full size in/out/through
Replaceable batteries | Rechargeable battery, not replaceable
Nudge :) | No nudge :(
2 synth, 4 drum tracks| 2 synth, 2 midi, 4 drum tracks
internal speaker | no internal speaker
Still fun to use | also fun to use

Engine seems to be pretty much same sounding. I think you can't still can't go wrong if you got to get original Circuit somewhere cheap.