The subframe change on the TBirds came along with the 6 speed transmission, and powdercoated engine. 1999 and 2000. Oficially there was not a Thunderbird in the US Triumph catalog in 1999. BUT some were imported late in the year anyway. AFAIK
all of those sold as 1999 in the US have the 6 speed and lower rear subframe, same as all of the 2000 and later.
TBS subframe is similar to the early Tbird subframe, except it is angled upward slightly, giving part of the increased seat height. The rear shock is longer and has a bit more travel so that also contributes to the higher seat height.
I've had an early TBird, still do but that one is in pieces.
The seating position of that bike and the TBS are very different. It depends on your personal preference, and your riding style. I have done 978 miles in 24 hours on my TBS once. I have done several 500+ mile days. I don't have a Stock seat though...
If you like the riding position of a Sprint you'll probably like the TBS better than the Stock Tbird, especially if you are comparing to the later Tbird.
The suspension is more than worth the difference in price between a TBS and Tbird.
It simply is better quality from the git go.