The synth/arranger debate will never end.

To help you decide, you'll have to work out which capabilities are required, say in terms of sounds, backing, playback, and keyboard

SOUNDS

If you want "standard" sounds (pianos, strings, brass, organs, percussions) then a good arranger or synth should be fine. Arrangers contain selections of "synthetic" noises too. If you want to "tweak" sounds a bit (brighter/darker) then arrangers can do this also. If you want to invent from scratch you'll need a synth or a high-end arranger.

BACKING

If you are composing tunes from scratch, then the "backing" capabilities of arrangers and some synths (in terms of hold down a a chord and listen to the band) are also less of a factor in absolute terms - but can be great for ideas.

PLAYBACK

If you want to prerecord arrangements and composition (using a PC?) then the midifile playback capability of arrangers and some synths will be useful, although the constraints of Standard midi files may be limiting. [Some synths and arrangers may let you use more than 16 channels of sound via track assignment to internal / external channels, e.g my Ensoniq SD1 gives me 24 tracks if I'm careful. It may only be possible to do this by editing sequences on the synth/arranger hardware]. I cannot comment on the edit capabilities of various keyboards because I edit my (cover version) midifiles on a PC. (If you dont intend to play live to your public you could go for a quality sound card and "input" keyboard for a PC and do it all using sequencing/ arrangement/ sampling software recording onto CD.)

KEYBOARD

As a classically trained pianist I lust after anything will 88 weighted notes. As a performer of pop cover versions regularly looking at upstairs gigs I use a PRS2000! If you have to have more than 61 notes, or require a decent feel, your choices will become restricted, but portability can suffer.

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These decisions are almost always a compromise somewhere. You could wait for the "perfect" keyboard - that debate rages on this forum at regular intervals!