We mow acreage every single year, we use a Ford/New Holland for the most part, but our riding mowers see 5-7 acres every week.
We have owned 3 Murrays with the 14.5hp briggs single cylinder. We own a Huskee (Tractor Supply) with a 15hp Briggs, and a Snapper with a 20hp Vanguard twin.
To sum things up, Briggs has always felt like a middle of the road engine. Ours need the valve lashed every couple years, carburetors need cleaned generally every 4 years. The solenoids on the carburetors bowls have failed a couple times (we just cut and remove), and two started smoking (lightly) after 5 years.
Here at the house every push mower has a briggs. We got 6 years from one before it became a major oil burner, and we have a couple going on 4 years. Just middle of the road, decent engines.
Now Kohler, our personal garden tractors 79 John Deere (12hp kohler) and 78 International (17hp Kohler) run Kohlers. They have been used their entire life and just need a tuneup every so often (points, carb clean, plugs).
Then we have a John Deere 94' model with a 15hp briggs we used for 14 years mowing the acreage with the briggs mowers listed above. Hydraulic lifters (no adjusting), never had carb off, and starts every time just touching the key. Has been retired to a friends house where it mows 1/2 acre every week.
From my experience Kohler makes one fine engine and never gave us a headache.
Briggs is fine, but don't expect to hand it down to your grandchildren if its worked for a living. That's my 2 cents.
My grandparents own a 30 year old Murray (pushmower) with a Tecumseh. Starts first pull, looks like [censored] but runs like new. Maybe had oil/air filter changed a couple times in its life. Haha
We always change our pushmower oil every season, clean as a whistle today when I checked. Our hard working riders get changed twice a year, usually Castrol GTX 10w-40.
So really, you don't know what your going to get or how long it will last. Just take care of whichever you own.