Eric I would lean towards the 300S too against the LGR but only as a collector first and shooter second. I feel that a well-cared for 300S, once it's been fitted with new seals/piston bumper, is a faultless performer that will last for tens or even hundreds of thousands of rounds without further maintenance.
The LGR's design is quite a way more advanced than the FWB, making it a superior rifle in purely functional terms. If you're planning to shoot competitively it will probably help you obtain better scores. But it's an SSP and while beautifully built, is inherently more prone to air leaks over time. Apart from a tendency for the exhaust seals (and piston seal?) to fail after much use/time, I've also heard that the little breech loader detent is prone to popping open when worn.
All the major service parts are currently available for both rifles so in practice there's no reason why you shouldn't be able to keep either one in working order for the foreseeable future. Although the mechanisms are very different, I'm guessing the actual work involved in maintaining each of them is roughly comparable and well within reach of a competent amateur gunsmith. There are guides to dismantling and repairing both 300S and LGR available on the internet. If you have trouble locating them, let me know.