You'll likely find the TDT a lot easier at autoparts stores or WallyWorld. I've only seen it in quarts; that does not mean it's not available in other forms, though.
The Delvac 1 is only available in gallons or pails, as I recall, and I've never seen it on the shelf anywhere but a few select truck stops. There are actually several different Delvac 1 versions (ESP, non ESP, Marine, etc).
Of the common ones you'll see, I'll compare/contrast the ESP and the TDT. They are not exactly the same product; but they are darn close. The TBN is different; the vis is a bit different. None of this matters though, as they both have capabilities well past any application you'll likely put them through. You might as well choose the one that is easiest to get and priced where you're comfortable.