I prefer the Verado. I have a 2017 200 Verado Pro myself. Quick answer for me = VERADO.
With the newer Verado, the thing that stands out most to me is the alternator amp/wat output is much better on the Verado. If you will have a lot of power draw for accessories or future add ons, that is a very nice feature on the Verado compared to the 4S. This alone would be a difference maker for me.
The 4S gives options on type of steering (hydraulic or power) and controls system (mechanical or digital), where the Verado has baked in top tier of both without options: power steering and digital controls DTS only.
In my mind power assist steering and digital controls are the only way to go; especially with Mercuries systems. So I like them baked in automatically with the Verado’s. Mercuries digital control system and DTS throttle are awesome!!!
You also have a slightly different gear case ratio between them: Verado 1.85:1 v 4S 1.75:1 I believe this is what helps give the Verado a better hole shot and acceleration compared to the 4S when proped properly. I believe this impacts torq, rpm range, and prop pitch options. That said, this is NOT my area of expertise. Others in here can speak better to the impact of this difference.
For me the alternator output increase on Verado is a big selling point. And since I really like the power steering and the Mercury digital controls and DTS system, I would be adding them anyway with the 4S anyway. Hence, I might as well have it baked in with the Verado.
As already stated, the Verado is the same powerhead as the other options. In the past, Verados were all 2.6L inline 6 supercharged powerheads, but now the 300hp models and below are all the same 4.6 V8.
250 Verado is going to be more expensive but have the fancier midsection, DTS, integrated electric steering and some other bells and whistles. It will also be much heavier. My guess is it needs the 1.85 gearing to account for the extra weight. Cadillac of the group.
250 4s will have zero bells or whistles but still an excellent motor by all accounts. As Viking noted, the 1.75 gearing will give you less holeshot but slightly more top speed (a little less than 200 more RPM at WOT).
One other model to consider is the Pro XS which is a standard 4s with a higher rev limiter (6200 vs 6000) and appears to be the lightest in the group. So it "should" give you another mph or two top end vs. the 4s and a little better fuel efficiency from the weight savings. The tradeoff from what I read is it's also a little louder so some of the weight savings may have come from the sound attenuation.
I have not owned the 4S or XS Mercuries, so not great on first hand comparative. I can speak a lot about my experience with our Verado these past 5 years.
To a point above, one feature I forgot to mention above in another reply. The Verado is significantly quieter. It is amazing quiet for a larger motor. However, as also mentioned above, more parts involved in them. They provide I think a better experience and better performing motor for the most part. However, more opportunities for issues or repairs as well.