re·is·sue
v. re·is·sued, re·is·su·ing, re·is·sues
verb, transitive
To issue again, especially to make available again.
update
verb
To make modern in appearance or style: modernize. See new/old.
I dunno, I think "update" does imply improvement. Unless you're updating to Windows. :-p
But seriously, everything that is "updated" does improve, or at the very least change a little bit. Your car example - yes, the principle is still the same, the engine is still relatively the same, but people have added more cylinders, valves, belts, gears, electronics and whatnot over the years. Really, each addition makes it an update.
Usually, when Fender makes a "re-issue", they try to stick to the original as close as is humanly possible. They made a few mods to the newer version (I avoided "re-issue" AND "updated" on purpose there), so technically it isnt a true reissue. The argument could be made that if something has a slight modification (like a rectifier tube, or a different gauge cable, or a different speaker model, etc) it isnt a reissue, but we're talking something big here. There apparently (I don't own either Model T) is a difference between the two, and I do agree with schrag - personal preference does play a big part - what you like depends on how you want to sound. If you want higher gain without the pedals, then the newer version is yours. if you dont need/want the extra gain or like first run amps, the older version is for you.
I think the whole "purist" mentality between Sunn and Fender/Sunn amps is kinda skewed...my beta lead was made just after sunn was acquired by hartzel. does that make it not an original sunn? What about those that Conrad Sundholm produced before the company was officially "Sunn"? See what I'm saying? There's no end to the divisions.
My final thought is that we are here because we all have amps that say "Sunn" on the front, and they all kick some major ass.