2286cc petrol/diesel, overhead valve, 5 bearing crank: In 1980, Land Rover finally did something about the crank failures which had plagued its four cylinder engines for 22 years. The new crank was so strong that Land Rover could (and did) get away with using the same crank for petrol and diesel engines. These engines lasted beyond the end of Series III production and into the first couple of years of the new Ninety and One Ten ranges.
Well worth paying extra for, if you are looking to replace a worn or broken 3 bearing engine in a Series vehicle. Crankshaft rear oil seal design is also far superior to the 3 bearing and easier to replace if it does leak.
RH's first paragraph speaks of the development path done in 1980, why this was done, and the result of the improvements. LR found the resulting crank stronger etc etc. These results came to form the future model line-up in that era. That'd be what LR did, conclusions they came to, and how these improvements were used. No opinion from RH. This is not RH's opinion, yours or mine. Purely an account of development, why it was done, and outcome within LR.
In the second paragraph, RH then expresses his opinion. Such developments are "Well worth paying extra for". Most would see it reasonable to agree. Thus, given choice, we'd take the 5MB. However the "I've had my 3MB for 40 years and I've never a crank go etc etc" set won't see the need.
Here's pix of the improved oil-seal RH considers "far superior". Here it's sitting in a modded clutch-housing to allow this seal easy use in a series. I can show, perhaps with another improvement over the 3MB, the 5MB's inertia-starter.
However, it just as likely, the "I've had my 3MB for 40 years and I've never a crank go etc etc" set will not see the need for this either. No need for 5MB. A 3MB is as good? And from their standpoint, they would be correct.
This becomes circular logic from hereon in...