if you were playing the D-50 for long time.then I think you will probably prefare Roland's products.so I suggest you go for the XV-88. if you want a synth with a composer and sampler from Roland I suggest you wait for the next workstation from Roland, other than that you got the XP-80/60 which only have the Composer. because I think they will include that.if you heard the KORG before then don't wait and go for the TRITON at once.you will love that keyboards (I own one).it have a composer and sampler and many other functions that you will like.also check out the korg Karma. all Roland's XV and the XP have expantion slots.
and the korg triton and karma too.
so what I am suggesting? here they are arranged for you.
1-Roland XV-88
2-KORG TRITON
3-KORG KARMA
4-Roland XP-60/80/30
hope that helps you
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vic