claudiu, if you need a instrument to provide all the accompaniment you want, you go with a arranger keyboard. the possibilities are wide, but esentially you work with the presets in the accomp area.
you would use this mainly if you perform live, by your self.
a workstation means an instrument providing more possibilities to tweak, to change, to use sounds in your own finger recorded tracks. you get also some tools you don t have with a arranger, for instance, arpeggiator, better sequencer, etc.
you would use this mainly for working in the studio, or making your own very original arrangements.
conluding, the arpegg. is a tool that belongs to workstations, not arrangers.
hope it makes sense to you!