You have to hear it.
SST series is “better” sounding in opinions of many, mine included.
However, the argument of “the more power the better” is also very valid.
To answer your question “Is the sonic difference between the two units fairly noticeable”, yes, it is, as far as you are comparing apples to apples, e.g. 4B ST versa 4B SST in the same system (I heard both in multiple systems).
Base on that, I would suspect that 3B SST and 4B ST would show even bigger difference.
But all this means nothing when discussed in isolation.
ST can fail miserably sonically in your system (meaning you will not be happy), with your pre-amp and speakers, or it can perform exceptionally well.
I have heard both situations, where either 4B ST or SST were producing heavenly sound or were very boring and mechanically sounding.
In those situations were it was not good, the “culprit” was not simply the speakers or the amp.
For example, I auditioned 4B SST driving Wilson Sophia II, did not like it a bit, yet Sophia’s sounded very good with McIntosh 500W/8ohm monoblocks (I am always talking in the context of the same system/room/time). This told me something about advertised efficiency of Wilson speakers and claims to be tube friendly, as well as synergy.
On the other hand, 4B SST worked very well with PMC OB1 and IB2.
Another example, 4B SST and 14B SST, driving a range of B&W speakers (704, 804, 802), did not like it at all.
But the same 4B SST with PMC OB1 and IB2 was very good. And 14B SST with IB2, that was some performance.
If you cannot audition and must buy blind, list the rest of your system, someone with direct experience may chime in.
Otherwise, going by specs, or by what someone says out of context, is shot in the dark.
Finally, with the caveats mentioned, your best bet is 3-4 years old 4B SST, as it appears to be within your budget, and you decided on Bryston in that category.
