I have a '77 D-35 that I have been using the Martin phosphor-bronze on for some time now. I've bought some others but never found anything else that lasted any longer or sounded any better than the Martins (I haven't tried the Newtones however). I did change a number of years ago from medium to light weights and that seemed to brighten the D-35 a bit (that model is notoriously bass-y and boomy due to the body size). I probably sacrificed a little life by the weight change but the bass lets the treble blend better.

When I start needing to buy replacements I always look for sales on the higher rated strings and invariably the Martins come in as cheap or cheaper than the supposedly superior strings. I shop internet almost exclusively since the nearest music store is pretty far away in high-traffic areas that I avoid if at all possible.

SC