"- arranger keyboards allow you to very quickly make a song using the automatic patterns, and record the notes to your DAW via MIDI, all parts (drums, bass, accompaniment, melody etc.) at the same time, and - depending on the level of the arranger keyboard - you get sounds that are about the same of synths (the top ones, like Yamaha Tyros or Ketron Audya, arguably sound better than some synths). The downside is that usually their synth section editing parameters are not as many as you'd find on a synth, not even close.

- synths allow you to sculpt the sound exactly to your specification, and create new, even exotic ones via a myriad of parameters. They also are usually expandable with cards / boards / samples. But you have to record each part separately on each DAW's track, so you can get better sounding songs thanks to sounds that are more customizable, but the downside is that it takes much longer to compose the song.

In short, if you are in a situation where you have to compose and create finished songs very quickly, an arranger is probably your best choice; if you can afford time in your studio with your DAW, I'd go with a proper synth.

EDIT: it also depends on your playing ability level: if you aren't yet at the level where you can proficiently play / program different instrument patches (drums, bass, synths, strings, piano etc.) in a variety of styles, then an arranger is going to be very helpful to you, because it will do the job for you, and you can learn from it. It's a quick way to create professional-sounding songs very quickly - simply press the Intro, Verse 1, Verse 2, Chorus, Ending etc. buttons at the appropriate moments in the song. The downside is that there might be other people out there who are using the same accompaniment patterns that you're using, so you run the risk of having a song with the same sounds and arrangement that someone else is already using..."