I don't think witnessed vs. not witnessed matters here. Sure, a witness signature is guaranteed authentic, but I suspect most forgeries are of "high value" signatures. I do not subscribe to Stan Lee's signature boosting a book's value by hundreds of dollars (unless he actually wrote the book in question), but some do. Unscrupulous people try to take advantage of that market with raw books. Even some pretty amateur forgers. A highly skilled forger might fool the authenticators. IDK how easy this is. In contrast, someone like Chris Claremont does a lot of signings. Just as one data point, recent sales on ebay have X-Men 101s CGC 9.4s going for basically the same price Universal vs. Signature. It would be kind of silly to try & forge CC's signature as a money making scheme IMO. Just spitballing, but maybe a collector is will to pay 10% more for a witness vs. authenticated signature, a reasonable guess for most creators. If the book sells for under $100 your talking about $10 difference give or take. Not big money.
Everything here should be about what you like/want, unless it's your business. Collecting isn't about profit. I am a collector, so I don't mind "losing" (a little) money in pursuit of building a fun collection. I do hesitate to invest more on a book that I, or whoever inherits my collection, could get reselling it someday. Basically making the threshold for deciding to sign/grade/both whether or not I would break even. I do have a few books that I don't think would turn a profit, but they mean something to me. I read a copy of this one to death as a kid & it was great to have Al sign & sketch this copy. There's nothing special about the comic itself, other than the fact that I like it.