Choices

The CR by a WIDE margin. I had one, and that motor is awesome.

Honda all the way

The CR! Having an 85' and an 86' both, lots of fun. Some parts can be hard to find but well worth the effort.

I like both brands, but if you have to choose only one from that era, it HAS to be the Honda.

The Honda was by everyone's agreement, and by a wide margin, the better bike. Both the '84 and '85 YZ125 were sub-par bikes. Not only slow and pipey (even with the Yamaha power valve!), but issues with z-spokes/hubs, ignitions, etc. The '86 CR125 was on the other hand pretty well acclaimed.

I had a new 1984 KX80 in 1984. In a drag race I could pull (not by much) my best friend's older brother on his 1984 YZ125. Of course, I weighed about 100 pounds and he was about 160 so the weight difference could have been the reason.

On the other hand, Hondas of these years ALL rot the outer clutch cover. A guy from Australia cast some replacement covers, but these were like $430 and I'm not even sure if any are left or if they would even fit your bike. Yamaha is the all-time champ at parts availability and interchangeability from the YZ restorations I've been around.

None of these bikes will ever be worth what it will require to get one well restored. If you've always wanted the Yamaha, do it. This should just be a fun project for you... Right? My own opinion is that anyone who thinks a stock 125 from that era is going to be fast is living a pipe dream. I had a 1984 KX125 in 1985 bought as a year hold-over, and even though it had won every shootout published that year, I remember the acceleration was a real letdown compared to the KX80.

Definitely get a 1986 CR125 but them I'm biased so don't take my advice!

Not an expert on either bike but I've heard from enough owners of the 84YZ125 and articles that the bike was just a turd. I believe even Eric Gorr has struggled with that motor. Just a bad motor. If you're trying to stay Ultima legal the 83 YZ125 is pretty good as of course the Honda CR125 or an 84 Husky CR125. Shit, all the 83 model Ultima "qualified" bikes are good.