jpapas,

almost all arrangers will allow you to split and layer, and most will do it easier than the regular synth - for example, Tyros allows you to layer up to three sounds for the right half of the keyboard, and a separate sound for the left hand. Roland (G70) allows you to split the keyboard into up to three zones, and layer two sounds for the right hand, two for the left hand, and have an additional sound for the middle section. I think Ketron allows up to 4 zones, but they have to be set up ahead of time (as programs). Some of the lower-end arrangers will not play the left hand sounds unless the auto-accompaniment is going (though you could mute the accompaniment part), but I don't believe this is an issue with any high-end instrument. Of course, any arranger can be played in the full keyboard mode, where you do not use splits, and simply play the right hand voice(s) across the entire keyboard.

Keep in mind, though, that for the most part, the arrangers are made to play back the built-in sounds, and any high-end arranger will do it very well, with large quantities of very high quality sounds. While some arrangers (especially Korg PA1X) will allow you to create new sounds, that is not the primary purpose of the arranger, so sound-editing tools are usually buried in the menus. The synths, on the other hand, make emphasis on creating new sounds, and will usually provide knobs and buttons right up front to make this task easier. Keep this difference in mind if creating sounds is a priority for you. Some new arrangers let you expand the sound set via expansion boards, or sampling, but for a die-hard sound creation enthusiast the arranger capabilities may fall short.

On the other hand, if you like to play and not to tweak, arrangers are way more fun, and will provide you musical gratification right out of the box.

Good luck,
Alex
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Regards,
Alex