To answer your basic question, there is nothing about older HW 35's that makes their performance inherently superior to a new one. HW has also maintained a very high standard of wood and metal finish on new ones. I wouldn't pay more for a vintage example than a new one, unless it was a discontinued high-end variant or otherwise exceptional.
But...I WOULD buy a good used one at a good price, too! You want to ask the basic due diligence questions about condition, history, and performance of course, but the 35 is a quality piece and its basic mechanical design has been pretty constant since the late 1960's. Action changes since then include adding a safety (late 70's), going from 13mm to 11mm scope groove spacing (late 70's), plastic piston seal replacing leather (early 80's), and reducing the transfer port diameter (I don't know when that happened).
That being said, Mycapt65's excellent comments on past construction glitches are very true; HW did have some QC issues in the 70's and 80's I think. But assuming you've sorted those...why not buy a 50-year-old gun? Wood and steel don't evaporate over time...springs, piston seals, and breech seals are easily renewable...and that's all there is to a spring-piston gun.
The 35 may not be "collectible" in the sense of being super rare or worth thousands, but it is VERY collectible if you are into history and variations! Over its life - about SEVENTY years - it's been sold under different brand names; had four different trigger designs (and many minor variations of the ultimate Rekord); had different barrel lengths; and seen countless changes to accessory bits like sights, trigger guards, sling swivels, buttplates, etc. Also some interesting limited-run specials like the ether-boosted EL 54 "Barakuda", chrome and gold-plated actions, and more. A friend of mine collected 35's and had 30+ different ones.
Evolution of the stocks is a whole separate game. For years three designs were offered - basic Sport (always beech, not maple), Luxus, and Export (both European walnut). The Luxus went from a rounded, traditional German Bayern style, to an "Americanized" somewhat squared-up Monte Carlo design in the late 70's, the other two evolved in detail over time, and a beech thumbhole was offered for several years. Also a few short-run wood variations like a green laminated "Safari" model, and a rare factory Tyrolean. And plenty of 35's out there that have gotten custom attention from tuners, engravers, and stock makers.
Here's my two, a Bayern Luxus and an Export with 22-inch barrel. If anyone has some 50-year-old "junk" like this they wanna get rid of, definitely give me a call...
Last but absolutely not least, Danny Garvin's awesome "Vintage Airguns Gallery" site is always worth a look when any older model is under discussion. You can get a great sense of the 35's history here:
Weihrauch HW35
forum.vintageairgunsgallery.com
