Caveat: I haven't built either of them, and I've never seen the Imai one outside the box. But I did buy the Revell one once, and I'm somewhat familiar with both of them through pictures. So I'll offer an opinion: the Imai kit is a serious scale model. The Revell one isn't.
The Revell kit was originally issued in the late 1970s. (I'd have to check Dr. Graham's book, which I don't have in front of me here at the office, for the exact date.) [Later edit: I checked the book; my memory was faulty again. The kit was originally released in 1970. Sorry about that.] That was a time when Revell was having some serious problems, financially; lots of people thought the company was on the verge of going bust. I was working in a hobby shop at the time, and I vividly remember a piece of promotional literature we got about the Revell "Spanish Galleon": "We've zeroed in on the market with this one: young married couples and interior decorators." Note the absence of any reference to scale ship modelers. The appendix to Dr. Graham's book notes that the "research" for the kit was done in the MGM movie studio library. That says a good deal.
The proportions of the hull are highly questionable, the proportions of the spars even moreso, and in general the...thing...looks like what it is: a decoration for a mantlepiece in a household where nobody knows much, if anything, about ships. I personally regard it as one of the more abominable creations of the plastic kit industry.
The Imai kit, on the other hand, gets rave reviews from just about everybody. Most Imai kits do. They're generally regarded as just about the best plastic sailing ship kits ever.
It needs to be acknowledged that the reliable primary sources about Spanish galleons are just about zilch; nobody really knows much about what ships looked like. But the Imai interpretation is, to my eye at least, about the most reasonable-looking one that any model company has come up with. Again, I haven't built it or even bought it. But on the basis of what I know about it, I certainly recommend it in preference to the Revell kit.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.