Richard,
The basic explanation, quickly :
-The X400 you keep referring to was actually a very short lived model number from the early Guild days, before "standardiazation". There aren't all that many X400's around.
-Your T-500 is a recreation of a fairly rare Guild guitar : the thinline version of the Guild X500. The X500 was the top of the line Guild electric archtop, laminated b/s, lam spruce top, maple neck, deluxe appointments (multilayer binding, bound f-holes and headstock, gold hardware, "'G-shield" headstock inlay, ...) The T-500 was a short lived thinline version of that guitar, that eventually sort of evolved into the Duane Eddy model.
-The X175 was the "standard" version if you think of the X500 as the "Deluxe" or "Custom". Same basic guitar except the neck was/is mahogany, chrome hardware, less binding, non-bound headstock, "Chesterfield" inlay on headstock, ....
Any vintage Guild will crush any NS Guild. You know, in my opinion. :lemo:
That's a bit of a broad statement. That all depends on your use, needs, taste, etc....
Given a choice between my precious '62 X175 and the Newark Street version (I also own), sure, no-brainer. the old one by far. As we all know, the comparison isn't all that fair - the old one was built at a time wood and labour were comparatively cheap, was not a cheap-ish budget guitar built to a strict price point when new, while the NS Guilds are a different story altogether.
But for the music I play, a later mini-humbucker or Westerly full-size humbucker version wouldn't work very well, and the current Newark Street X175 actually works better because of the single coil pickups. For the tones I go for, gotta have the single coil pickups.
And I'm super-spoiled now, owning four nice Hoboken-era single coil Guilds, I have more than I need. But as a late teen into Rockabilly, I would have been
over the moon with the NS X175 - a well built single coil, Bigsby equipped hollowbody that sounds and looks the part for a price two summer jobs would have taken care of? All I could get/afford at that time was maybe a heavy, overbuilt Gibson ES-175 copy with painfully inadequate humbuckers by Westone or Ibanez or Aria. (I was a teenager in the 80's)
And given the price and "replace-ability" of the NS X175, I'm still really happy with that guitar. Flying gig where I'm not sure if I'll be able to take the guitar on board? (most times you can't any more) Handy gig bag backup for gigs closer to home? "Expendable" guitar for those couple of rowdy/outdoors/biker gigs a year where I feel kind of weird about taking my precious vintage guitar? With the couple of simple tweaks I've done to the NS guitar, it sounds the same as my old X175 to 99.7% of the crowd, looks the same too, if a little newer and cleaner, and it plays great, stays in tune, plain works well for its intended purpose. If it gets destroyed by an airline, gets stolen or damaged, I'll curse loudly, shrug and go "oh well..." and move on. Not quite the same as with my vintage guitars.
My NS X175 is not as wonderful and fantastic for my needs as my '62, but neither is any Westerly Guild I've played. So there. Horses for courses, or something like that.