This can be looked at several ways...
1 - From the standpoint of wear, the more parts that move the more potential for wear there is, so keeping the chronograph stopped in both a horizontal and vertical clutch means less parts moving. But IMO the wear is not terribly significant in either case, so if you want to let it run, let it run and maybe replace an extra part or two at service. For a horizontally coupled chronograph, in most cases the wear and tear happens when the chronograph is started, since you have one wheel that is constantly turning engaging with another that is stationary:
The teeth can impact each other - this is why these teeth are very pointed to minimize damage:
2 - From the performance standpoint, more parts moving generally mean more load on the movement, so a lower balance amplitude will result. Amplitude loss can affect timekeeping, and how much amplitude is lost depends on the design and condition of the watch. For a horizontally coupled chronograph like the Speedmaster Pro (321, 861, or 1861) Omega allows the amplitude drop with the chronograph on compared to off to be as much as 40 degrees. This will certainly have an impact on timekeeping, and I am usually looking for more like 20 degrees drop rather than 40 personally. In a vertical clutch chronograph, the coupling is accomplished in a different way, so here is a chronograph wheel from a Cal. 3301 family watch
As I've written before, the brass coloured parts are part of the main wheel train that allows the watch to tick (so that wheel is running constantly when the watch is running), where the steel parts are part of the chronograph. When the chronograph is stopped, there is some friction between the steel and brass parts in this assembly. When the chronograph is started, the whole thing turns, so that internal friction is eliminated, but there is still load created by the whole assembly turning between 2 jewels. The coupling action doesn't cause any damage though, and is done via 2 "arms" (called the clutch rocker and clutch lever) that swing in and out to raise/lower the dish shaped part above, that is spring loaded. Here is the movement with the chronograph stopped, so the arms are swung in:
Here is it with the chronograph running, and the arms are swung out:
So what is the difference in the load between these two? I can't measure that directly in terms of torque, but I can see the difference in amplitude. Here is the timing machine with the chronograph off - note the balance amplitude is 299 degrees:
And here it is with the chronograph running - note the balance amplitude has dropped 5 degrees:
Now once per minute, the amplitude drops further as the minute counter is triggered. So it's basically a wash on this one.
Now regarding the advice to leave the chronograph running
all the time on the 6139 and other vertical clutch chronographs, I would call that very bad advice actually. In addition to everything I've noted above, there is another consideration. If you constantly leave the chronograph running, there is a chance that the chronograph wheel's 2 parts will seize together. I have encountered this on several vintage Seiko's, and even here on this Omega - watch as I start off with the chronograph stopped, so when I push the operating lever to start it, the movement immediately comes to a halt:
It starts up again as soon as I turn the chronograph back on. This required replacing of the chronograph wheel, which is not a big deal for a modern movement, but for the vintage Seiko's these are discontinued and getting hard to find. If you want to run it most of the time that's fine, just make sure you run it with the chronograph stopped every so often to keep the two parts separated, and if you are buying one, make sure it runs with the chronograph switched off - if it doesn't look for another example.
Cheers, Al
