I had both in my system almost back-to-back about 10 years ago.
The 9Ts were equipped with the battery power supply and were very nice amps. At the time I was moving away from the model 12s due to an overall lack of dynamic drive (which the 9Ts had in spades). Overall they were on the warm side of neutral, but not at all slow or plodding. My only issue with them (and ultimately the reason I decided against them) was a bit of upper-mid to high frequency glare that was present when the system was pushed. I never had this problem with the 12s, but with the 9s I started to suffer fatigue after a while.
The 9Tis were equipped with the upgraded switching power supply. They possessed all of the dynamic drive of the 9Ts, but with midrange performance that was much more like the model 12. Best of all there was none of the glare that was present in the 9Ts. They were one of those rare components that drew me into the music and caused me to stop thinking about the stereo.
I'll throw out one caveat here and that is that at the time I had Wilson W/P 6s and they tended to have a harsh top end to begin with.
Keep in mind that from outward appearances the various versions of the 9s were identical, but internally the 9Tis were vastly different from their predecessors. In addition to the transimpedence circuit design they used completely different output devices and board layouts. As such, there was no upgrade from the 9T to the 9Ti (although at one point a 9Ti could be upgraded to a 9TiHC - the high current version).
IMO the 9Ti was the absolute pinnacle of the Rowland lineup for the subsequent 10 years and none of the newer products 201/501/3xx could hold a candle to them in overall musicality and performance. Unfortunately, they were absolutely massive and somewhat difficult to live with both in terms of size and heat output.
The newer amps 625/725 appear to be returning to the sonic roots of the model 9s with some notable improvements in sonics (better resolution and lower noise). As for the 825/925 no one really knows yet as they are a different circuit design from the 625/725. According to Jeff they take the good qualities of the 625/725 to a much higher level... ask me in a couple of weeks and I should be able to tell you