HI,
The differences between the two instruments are starting to blur. I own the Tyros 2 and the Triton Extreme and I just purchased the Korg M3. The main difference between the two is that the arranger keyboards have preset arrangements that are button triggered and contain variations for those arrangements to add intros, endings, and fills.
With a synth, the drums and bass parts are generated by the sequencer which must be programmed by the user. Althought the sequences can follow chord recognition, they are not as dynamic as those on an arranger.
The exception to this is on the new M3 synth from Korg. The M3 has Karma technology which actually generates grooves from the chords and base drum sequences that you play. It actually creates many complex variations on 4-5 tracks of music including the base line. It does have chord recognition as well.
So if you are a composer you would probably be better served with a synth since it has more creative control, whereas the Arranger really is for performing live cover songs.
Hope that helps.
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Al Giordano
http://www.arrangerworld.comTyros 2, Yamaha P-250, Korg Triton Extreme 76, Roland VK8-M, DW Collectors Series Drums, Roland SPD-S.