I owned several Mass. built bikes and currently own one Sarratoga built Yo Eddy.
In my experience the finishing and detailing are about the same and the paint is just as nice, plus the frames benefit from years of evolution, lighter tubesets and nicer droputs based on the ones developed for the 10th Anniversary.
My first Yo Eddy had welding that had an almost fillet brazed quality, the Sarratoga frame is not quite as nice but still excellent by most standards and about on a par with my Sommerville built Wicked Lite, after all these bikes were built by Serrota who have a reputation for building some of the most highly regarded U.S road bikes.
Having said all that I still like the Sommerville frames best, but the appeal is as you say emotional rather than objective..
I too own both MA & NY Fats, hard to say on the paint but painting regulations changed a bunch in the mid to late 90s. I prefer the domed stays(MA) over the welded on domes(NY) but how/if that affects the ride I can't say. I also liked the crisper folded downtube gusset(MA) over the gusset cut from a tube(NY). The welds look comparible to my eyes. One has to admit that Serotta certainly puts out a quailty product that is a real testiment to their skills. Heh, I just love riding them, and the passion that seems to have a grip on us all.
Just my 2 cents...
You are saying that on the older Mass. Fats those ends were made by manipulating the tube and on the newer ones those ends are welded on seperate parts. My Yo is from NY, but I can't discover welds, so I guess the Serotta people removed the welds by polishing, right?
On the downtube gusset: I assume this should be the crisper folded one on a Mass. Fat:
Sorry this took so long.... on my Mass. built Fats the stays are formed by heating and shaping the tube into a point, on the NY built Fats the tube ends have the ends welded in. On the down tube your pic shows the style that is on my Mass. Fats where as the NY Fats have a piece that if flat to the tube, like it was cut from another tube. Little differences, but just stuff you notice.