I have two Nikon scopes sitting on my workbench, and I'm trying to decide which to mount up on my rifle for long range work (800 yards plus).  Need some help deciding, because on the face of it there is little to choose between them.  (Please don't try to convince me to buy yet another scope - I have these and I don't have money to spare for anything new right now - so it HAS to be one of these two).

They are a Nikon M308 4-16x42 BDC 800 (older model on a 1 inch tube), and a Nikon P308 4-16x42 BDC 800  (newer one on a 30mm tube).  I know the Monarch glass on the M-Series scopes is supposed to be better than the Prostaff glass on the P-Series (not quite sure how, darned if I can see any difference, but that's the commonly accepted wisdom).    There used to be a number of other differences  but Nikon recently updated the P308 and now as near as I can tell the only real difference between the two is the range of adjustment. The P-series scope being on a 30 mm tube has 90 MOA of adjustment, whereas the older M308 only has 40 MOA (new M308s on the 30mm tube apparently have 60 MOA - but that isn't what I have).

Which do you recommend?  Do I go with the greater adjustment range, or the supposedly better optical quality?  I already have a 20 MOA mount, so not sure if the larger adjustment is really the deciding factor.

Go with the M-308 4-16 BDC 800 scope for the longrange rifle. The MONARCH glass is better and will prove itself when you start reaching out past 400 yards.  Both scopes have the much needed parallax adjustment required for optimal shooting. The 20 MOA base will defintely help. Check out Nikon's Spot On Program for adjustments needed and to see what distances you will get form your BDC circles. The new P-308 with the 30 mm tube is a very nice scope. You are right that it has lots of internal adjustment and it does have upgraded glass coatings.  I hope you'll post a range report and pics when you get dialed in?
Have fun. Definitely try out a few types of ammo to see which works best for your rifles.

I have found the turrets on M series scopes to be more accurate and repeatable than the turrets on P series scopes.