American Tug and Nordic have more commonalities than differences. They were both designed by Lynn Senour, the legendary (and sadly recently departed) designer of semi-displacement hulls. There should be more Nordics around than ATs, as the Nordic company has been in business for a couple of decades longer (1980's I believe) than AT (2001).
The similarities are also due to common management backgrounds. AT was started by a group of Nordic executives who wanted to strike out on their own. Tom Nelson, Kurt Dilworth, and Mike Shoppert worked at Nordic for many years, and started AT.
In my humble opinion, both AT and Nordic are good boats, and both have their fans. The differences are more in subtleties and execution than philosophy. Nordic was designed from the start as a pleasure boat. The AT uses a commercial fishing boat hull, which I believe was formerly called the 'Bristol' trawler, designed by Senour in the 1980's (no relation to the "Bristol" trawler of the same name of the 1970's that I think was built in India). The AT group acquired the molds to the Bristol hull, and used that for the first model, the 34 ft American Tug (now called the 365). All of the other ATs since then have been based on the original Senour design.
AT still builds commercial fishing boats, in various sizes, based on the original Senour design and using the same hull molds as the pleasure boats (with a few being shortened to meet specific length requirements).
Again in my opinion, having crawled over both ATs and Nordics with a microscope, the AT is a little more heavily built, beamier for its length, greater freeboard (all reflecting its commercial roots). To my eye, the Nordic is a bit of a 'prettier' boat, and the AT more 'masculine'/workboat like. Both perform comparably well, which is to say outstanding rough water capabilities (albeit both are coastal cruisers and not ocean crossers). I found the AT to have a bit better speed and a more level, smoother (albeit somewhat wetter) ride, and a heavier layup schedule in the hull and superstructure.
Probably not coincidentally, the AT and Nordic models do not exactly overlap in size (with the possible exception of the AT 41/435 and Nordic 42, which are very close). The size differences among the models give buyers a wider range of choices, and perhaps intentionally not directly compete with each other.
American Tug goes to great lengths to sweat the details of their boats. Anyone that has crawled through the machinery spaces invariably comes away impressed with how everything is built to commercial workboat standards. The whole boat is significantly over-built (probably much like your Seahorse, Mark). Every important fitting, valve, and mechanical and electrical component is clearly labeled to make identification and service fool-proof. AT is also very willing to customize a boat and made customer modifications; there was a page of dozens of changes and custom features on my current boat that they were willing to accommodate.
It's a straightforward, very sturdy, extremely well-built, seaworthy, and reliable boat that just inspires confidence in its capabilities and construction. My wife (who is not much of a boater, but supports my addiction) compliments the boat in saying that she feels 'safer and more secure' on our ATs than any of the other boats we've owned or been on. It's certainly not perfect (what boat is?) and is not the boat for everyone, but short of spending far more money on a custom build, it's the boat that fits us best.