Inherent Difference - 12AU7 and a 12AT7

Because these are different tubes and not really interchangeable it's hard to give you an answer. The 12au7 has a gain of about 19 and the 12at7 is 70 I believe. You would need to change the biasing of the tube to get it to perform to it's best. You could plug an au7 into at7 position and it should operate but not at it's best because of what I mentioned. 12at7 are often found as phase inverter/splitters. I commonly see 12au7 or 12ax7 as the preamp tubes in the first stage of amplification. Can't give you an exact reason why. I have used amps and preamps with both and it came down more to the brand of tube more than what kind of tube for me. "referring to these two tubes" Some people like the 6SN7/6SL7 family better and don't like either. I like all mentioned above. Don't know if I have helped you?

Well here is the issue; I have a Hovland HP-100 preamp and initially they used a 12AU7 in position 3 of the line stage. (Position one and two are both 12AX7's). In subsequent issues of the 100 they went to a 12AT7 in position 3. I can't even remember now which I am currently using but I need to replace them and am debating which way to go.

I love the Hovland but the tubes always get noisy fairly fast; after a year or so... I guess that while I am replacing the AX7's I could buy one AU7 and one AT7 and see what difference I could detect. I am just wondering if one or the other would help cut down on the quick degradation to noisy status...

Some info on the gain characteristics of the "12" family of 9 pin miniature tubes:(http://thetubestore.com/gainfactor.html) I would suggest some of the better/quieter, and longer lasting, European varieties(NOS- Siemens, Teles, Valvo, Amperex, Mullard). I've listed them, in order, from most transparent, to warmer/tubier. YES- Each manufacturer has a characteristic sound, in MOST gain circuits, regardless of the actual iteration.

Just because the company switched from an au to an at doesn't mean that you can. I'd bet they changed something or determined that more gain was needed and made the modifications. As Hovland before you even consider it.
As far as lasting 1 year, these tubes should last 3000 to 5000 hours. Unless you are leaving it on 24/7, you should get many years out of these. Even if they are wearing out, they shouldn't get noisy so easily. Usually your dynamics will just flatten out?