They are no longer the same. Delvac 1 is the one that lists all the big diesel specs, and the TDT is aimed for the HD pickup diesels. I think that Delvac 1 may be heavier duty and certainly costs 8-10 dollars more per gallon, possibly due to the more expensive components in it. The TDT is no slouch however, and has turned in some very good results, as did it's predecessor.
The CJ-4 versions of Mobil 1 5w40 TDT and Mobil Delvac ESP 5w40 are not identical, but they are close enough....
...unless your warranty requires an oil meeting Caterpillar ECF-3, or Cummins CES 20081, or Detroit Diesel 93K218 or 93K214, or Mack EO-O Premium Plus or EO-N Premium Plus 03, or MAN M3277, or Mercedes Benz 228.31 or 228.3.
I wasn't sure about the new formulas if there was a huge change that was made to the new Delvac. I would guess to carry all those ratings it had to be tested and that all adds up. So for us "little truck" drivers, either one will be a great oil to use and if you can find Delvac at a good price grab it up.
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.