Both are amazing airguns with their own pluses and minuses.
The first FWB recoilless springer match rifle was the model 150, made about 1963-68. The improved model 300 was made 68-73, and the ultimate 300S from then until the end of production in the 90's. All three models share most components, and accordingly their serials are all in the same sequence.
The model 300 is really the model that did the most to cement FWB's reputation - utterly unchallenged at top-level national and international matches in its day. During the long run of the 300S, competition (Anschutz 250 and 380, Diana 75, and the game-changing Walther LGR pneumatic) started to make inroads though .
The 300 has a longer barrel than the 300S, also a shallower stock (looks more like a sporter rifle, which many like), a longer cocking arm with side-hinged latch, a straight cylindrical barrel weight sleeve (not stepped at the front like the 300S), and a trigger with fewer adjustments than the 300S. All that being said, there are transitional guns with features of both; sometimes one has to rely on the serial and model number stamped on the action.
Being older and technically rarer, the 300 is harder to find in good condition, but the down side is some critical parts (especially trigger components) are no longer easily available. But I sure wouldn't turn down a deal on a nice one.
FWIW, I own a fairly early 300, quite a plain-Jane one with a beech stock. I had the gun professionally re-sealed and if not a beauty queen is an amazing performer - shoots very hard, absolutely dead accurate, and more tolerant of different ammo than my three 300S's.
The 300S is an incredible machine, though. The slightly shorter action, extra sight and trigger refinements, and wide variety of beautiful collectible walnut stock styles, make for a very satisfying shooter. It also has the very practical advantage of better parts and service backup these days.
As far as standard models, the 300 is much rarer, but I don't know that's it's worth any more to most shooters. The most collectible FWB's are probably the rare 300S Running Target variants, with special single-stage trigger and highly modified stocks. The 300S Mini, with its short unsleeved barrel is the lightest if that matters to you (I love mine!). Other special 300S variants were the "Universal" with more adjustable stock, the 300SL with special stock matching the FWB 2000 cartridge rifle, and of course any gun with a Tyrolean stock (available for all three models, but rarest on the 300s as Tyros had been banned from competition by then).
