Philr -- I, too, have both a Westerly D-25 (1974) and a G-37 (76). I've owned several other D-25s of varying years and even a D-4 and D-4-12. I bought the G-37 new and wouldn't give it up for all the tea in China. The D-25 is the second in line for saving, but it's a very different story - it was pieced together by luthier/repairman Holger Notzel from bits from all over - the body bought off Ebay without a headstock, etc. Both instruments are dreadnaughts with arched backs. The G-37 is maple back and sides stained to look like mahogany with a sunburst spruce top. The D-25 (FrankenGuild) is mahogany top, back and sides with a quarter-sawn mahogany neck. Both are fairly plain in their appointments. The sonic differences are that the G-37 is smoother with slightly less projection and volume, but still a ferocious sound. The FrankenGuild has a warm, slightly muted and dark sound with plenty of volume and projection. Both are almost too much guitar for some small rooms (for road work I use small jumbo GF-25s), but are amazing in the studio for depth and clarity and eveness of tone.
If you are looking for a live performance guitar - especially for solo work, I would definitely go for a D-25 - whether using mic. or pick-up. If you are going for strictly a studio guitar - the G-37 is the better, in my humble opinion. If you are looking for an all-in-one, both serve adequately, but I would shop around. Not all Westerly D25s are created equal and some are outstanding while others are simply adequate - soundwise. As far as the D-4 series - I found them only adequate, though nicely done. Very light weight and entry-level compared to the D25. Personally, I believe if there is such a thing as the flagship instrument for a brand, Guild's is the D-25. You can't go far wrong with one; you just pick your shade and go.
I hope this helps, but remember what I said somewhere near the top - if you take any and all of my Westerly Guilds away (I think I have 12 or 13, can't remember right off hand) leaving me only one - I would go for the G-37, it's a brilliant guitar. Second in line would be the D-25 FrankenGuild. All the best. dbs