Re: differences between the D-50 and D-55... aside from the obvious cosmetic differences, it will depend a lot on the era... both guitars (but particularly the D-50 in recent years) have seen changes in specs.
Someone else will have to comment on tonal differences in the Tacoma and New Hartford years. I have/had 4 rosewood Guild dreads from the Westerly factory, all with Sitka tops and EIR B/S:
A '76 D-50 (straight braces that had been shaved post-factory; the D-50 model got scalloped braces sometime in the mid-late-'80s);
A '92 D-55 (scalloped braces; scalloped braces introduced to the 55 in the early '90s);
A '94 DV-72 (scalloped bracing); and
A '95 DV-73 (scalloped bracing). The top bracing on the 73 is somewhat different than that on the DV-72 and D-55 from just a few years earlier.
All of these guitars have their own distinct tone. The D-50 has the complex character of rosewood, but is brighter, brasher, and in-your-face. The D-55 was (I say was, because I sold it last Nov) majestic... like a grand piano, with a gut rumbling bass. The DV-73 is brighter than the D-55, but a more refined and complex tone that the D-50 (almost an in-between tone). The DV-72, IMO, is the "best" (very subjective) of them all (at least of my limited sampling)... it has all the best tonal qualities of the D-50, DV-73, and the D-55 rolled into one... which was why I was finally able to let the 55 go.
So, if you are thinking/choosing between D-50 and D-55 (and I don't think you can go wrong with either), I would really suggest auditioning (a couple of) each, and see what you ear prefers... it may very much depend on what your ear is atuned to now, and the style of music you play.
Good hunting!