I wouldn't think of either one of them as "highly collectible". The BMW would probably continue to be a veyr reliable ride and if it's in as good of shape as you say, You could probably ride it anywhere. As far as price goes, I can't really say. I wouldn't expect it to be much over $3000 at the high end? Unless it's a show-room model bike or somehting.
Yamaha's RD family of bikes are very fun. The RD 350 and 400 have a pretty strong following among RD faithful. The 250 is a little smaller and not worth very much in money, but can be a very fun little bike to ride
The first bike I ever laid down in gravel was a RD 100... lol. I really, really liked that little bike and I made sure no damage came to it, but it sure burnt the crap out of my ankle!
Like Pig9r said, check some other places as well. I'm not an expert on either bike (or anything else for that matter...

) but I grew up working at a motorcycle shop and have been around alot of these type of bike.