My experience with the Sherwoods is that engines that run often, they seem to hold up ok. When sitting, the graphite/ceramic seal sticks, then the shaft spins inside the seal assembly. Then it leaks and it is all downhill from there.
The one on my Cummins lasted 10yrs and about 2500hrs before it died. I would put the SMX on but my custom motor mount that I am so proud of has a clearance issue with the longer pump.
I put a used takeout sherwood on that had a known good seal, it has been fine for about 400hrs.
If your sherwoods are not leaking, there is little risk of sudden catastrophic failure. Replace as soon as you see leak. The SMX is the way to go at that point.