Torecagli - You don't post what kind of music you intend to create. It might help us help you if you're a bit more specific about what you are trying to achieve.
In general (please remember I said this!!) arrangers tend towards the more 'bread and butter' styles rather than cutting edge hip-hop or electronic styles. This is not to say that they don't exist, but you are probably going to find more styles for schlager or waltzes than ragga or Bristol two-step!.
I think that if you don't REALLY like the arranger's ease of jamming and composing, you will generally have a bit more flexibility of use of a FantomX or MotifES, etc., but the initial 'noodling' while you come up with a song or idea is generally easier on the arranger.
I think in your case, hang on to the Tyros, and get a Triton or FantomX (or take a good look at what you can do with a computer and Reason or Project5 etc.). You probably don't need a Motif because you've already got a good selection of Yamaha sounds in the Tyros.
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An arranger is just a tool. What matters is what you build with it..!