Here is what I think about synths:

1) synth keyboards are basically keyboards that are dedicated for assembling and tweaking sounds. It does so by adjusting the filters on multiple samples and combining these samples to make that sound.

2) Usually, dedicated synth keyboards do not have arranger capability. In fact, not all of them contain a sequencer. But some do.

3) So it depends on what you want to do in terms of sequencing and performing. Do you want a sequencer at all? If so, how do you want to sequence, from scratch by playing every single note or use styles to help you build some blocks/tracks? Do you ever use Styles for performance?

4) But the most important issue is do you make your own "synth" sounds? Do you spend hours on creating sounds?

Here is what I believe about this. I personally like to use "accoustic" and classical sounds. Therefore, I won't be able to tweak a sound better than the manufacturer. Let the artists hired by the manufacturer do their research and come up with the best "acoustic" classical sounds. For other non-acoustic sounds, Any decent set will do in my songs and performances. So in reality I do not care about a synth.

So in my opinion, a fully programmable arranger with excellent sounds is the best deal. But there is a catch.

In the past, musicians always thought that synths are for professionals. therefore, manufacturers loaded their synths with their best sounds. This practice is not completely true but you can always see hints of this.