Errrrrrrrr, I'd keep the 999. Why? Because it's going to get ridden more being easier to live with in regard to repairing it, ergonomics, worry about ruining a collectors item etc. Also, unless it's a really special 851 it's value is likely to have levelled out and will only increase in line with other Classics. The 999 I'm absolutely convinced will continue to rise at a greater rate because of the constant comments in the bike press etc around how good they are and haven't been appreciated. Oh, and because the 999 is Yellow
I’d keep the yellow 999S. Great looking bike with handling to match. I think the 851 looks a bit of a munter by comparison.
because its beautiful and I like retro This maybe someone's here as I snapped it in Bristol Italian fest
A gorgeous bike or one with somewhat better performance? Its OK to have a yellow bike when you have a few, but if its the only one... (LOL) Now if by some freak thought process you end up with a flawless 999s in the garage, its a good problem to have. But I reckon you will always wonder what happened to your classic 851 and scour the sales ads looking for it. A great 851 is not so easy to find again.
What classic car? The “itch” that the car scratches might help simplify which bike you can more easily live without. Which bike do you use tend to use most, and which would you try and save first if the garage was on fire? I always said either my 888 or the ‘76 Jota, will be the last out of the door but as neither have turned a wheel in 4 or 5 years now, I’m probably due a significant rethink.
851, much more interesting and less mass produced. Can look sensational, although the pic above is an SP3 I believe (which I hope are a different investment proposition)
851, but then I am Biased. The 999 was a bit of a "marmite" bike looks wise back in the day, but it has become a bit of a grower (me included).