Hi, my Sterling is a "heavy red brass" bell but in 300mm (11.8") rather than 305mm (12") or 310mm (12.2"). The smaller bell gives a more compact sound and slightly better response, but the bell material gave me a bit darker sound. My Adams E3 is .70 thickness yellow brass and is about 12". This is the thickest brass bell that Adams normally does with the exception of the E2 which is .80. On balance, those that listen to me seem to like my sound slightly better on the Adams. From behind the bell, I generally prefer the Sterling sound. Go figure.
As regards valves, I cannot help much. My vintage Sterling is 2009 and he was still using Bauerfeind valves (same as current Adams). After 11+ years of ownership, the valves are like silk, though manufacturing crud (polish compound, etc.) inside the horn made them somewhat sluggish at first. After about a year of "break in" and several really thorough cleanings, they became markedly better. I consider them the equal of Besson or Adams now.
I know that Sterling has worked very hard with their current valveset manufacturer over the last few years to improve the quality. I would consider them to be pretty good these days. Probably the equal of any of the new premier Chinese brands. Also, the last time I spoke with Paul Riggett, he indicated that overall, the new horns are probably just a bit more responsive than horns of my vintage. The trigger mechanism has been improved, and there have been a few minor tweaks along the way.
Adams E3 0.60 Sterling bell - Prototype top sprung valves
Concord Band
Winchendon Winds
Townsend Military Band