Getting into Vintage

I cannot speak to the YZ but the RM was a great bike, proven engine from the 370. The swingarms were known to wear pretty badly on the drive side though. If the arm is not worn then I’d buy it. My experience has been that parts are readily available.
Pretty sure the YZ was good also
As with anything of that era, the tanks can be prone to cracking without warning but RM looks great with the earlier aluminum tanks grafted onto them IMHO.
Modern oils make about any ratio possible, especially on big bores.
Good luck

Bob Hannah replica….

I believe the YZ will be more reliable for you and have parts more readily available, both will be equally competitive as most vintage races are short motos on rider friendly tracks. The 1976-78 RM370/400's have a very bad habit of ignition failures and scarce to no OEM replacement parts to get spark again. Have a blast if you pick up either one and live in an area with VMX races nearby.

Get both!!!

77 YZ had a mild steel frame and swingarm, RM had chrome moly steel frame and Alum swingarm.

Must warn you....before you know it you'll have 10+ bikes in the garage :-) try Vape Ignitions out of Europe....work great on Bultacos

Both are great but I prefer the RM

If you want to ride vintage, I can only recommend Husqvarnas from 79-83. This range has a massive amount of support, that will keep you up and running easily within days.

I actually owned both of those bikes, the yz is much faster and way more reliable, the rm handles well, and run decent with a few mods, but I put three transmissions in the rm, several top ends, and constantly had to have the frame and swingarm welded. It was my first and last rm lol!

I factor depending on the organization you will race in is what class(s) you want to race in.
75-77 and 78 into the early 80s can be 2 different classes. So worth a look.

I owned a 79 RM400 mellow for a 400 mx bike all bottom and mid. Easy wheelies and always using a boot to start it with. Gave some nasty kick backs.

Guess the transmissions weren't the best. Guy I got it from said it was a 4 speed. No first gear. I had assumed at the time big bores didn't need a 1st gear and thought nothing of it. I rode it like that until I sold it. That was 30 years ago 😳

Guy I sold it too shortly afterwards exploded the rear hub. Ripped all the loose sprocket bolts out.
Got to see it happen.

Also had a 1980 RM125 with fox airs. I remember less about it. It was fast light handled well okay brakes. I got into woods bikes and bought this beast 1980 kdx250.

Yamaha's are by far easier to find parts for and often Yamaha ran a lot of motor parts for many many years in various bikes. I've owned and raced a variety of Vintage brands; Cagiva, Yamaha, Suzuki, Honda and Can Am. Yamaha was probably the most robustly built and easiest to find stuff for. Performance wise they all have their quirks and it's totally rider preference.

It all depends on how serious you want to get. If you're just out there to have some fun, you'll be fine on either bike, but if you want to push the limits of those bikes, you'll soon find out their weak links and suffer the occasional DNF and the week after the race you'll be chasing down parts from potentially around the globe. If you let it consume you, and are competitive and meticulous about things like I was, it can be pretty damn expensive, even with some sponsor help here and there. I went full tilt into vintage racing and absolutely loved it, met some amazing people along the way and ultimately have no regrets about that place and time in my life. Just make sure you're having fun, and you'll be just fine! Good luck!

I got out of vintage because local tracks and riding areas were more for modern suspension. I was limited to vintage only racing where the flatten the track a bit.
I also owned a 80 RM125, the bulges on the side panels from the shocks interfered with my riding boots (plastic calf guard) so be aware its different riding than modern stuff.