Yes, it is hard to tell without some maintenance history/documentation.
But, unless you have your heart set on the LS, was has a certain "cachet"
(though mono R65s aren't that common either) I think that , all else being equal (which it may not be), that
you may have a better chance of the mono requiring less costly maintenance by design, than the LS.
Now, if the Ls was well cared for and the mono was not, this goes out the window, but consider:
1. The mono should have the improved valve seats for heat transfer, so one wouldn't EXPECT to have valve/head problems down the road
with it. If no documentation of the LS valves/seats being done already, then assume it will be coming up at some point.
2. Input shaft on the later model airheads (sometime after 85, but can't recall when) BMW started to hard chrome the transmission input
shaft splines - making them a bit more durable. Still not properly lube and care, but are less "delicate" in that regard.
3. As you indicated, the mono bike shares the same frame components as th R80 mono, so parts for frame and bodywork are much easier to find and are more likely available - certain LS body panel parts are NLA. I think the same goes for the forks, shock, handlebars, etc - all are in common with the
R80 mono bikes, whereas the twinshock R65 models have shorter shock absorbers, and their own pannier mounts separate from the R80/R100 of the same period.
These things above can mean less $$$ in repairs in the near future, but again, without knowing the history or proper mileage of the bikes, it is
not easy to predict which would be better.
