That depends on what you want to do with your music.
If you dont need to sequence then the xp10 or the xp30. The xp10 you can get right at $500, and the xp30 you can get at about $1000
If you need to sequence then I would recommend the xp60 ($1300), then you have the sequencer, if you are a serious keyboard player then I really suggest the xp80($1645).
But I think you should go with the synth, you will thank yourself later.