It's a difficult thing to pin down as the technologies are so different. However what I will say, is that with both technologies it appears to be all about the power supplies. Given properly engineered power supplies it is hard to get a fag paper between the two methods, providing your output transformers are up to the job on the valve side of the equation.
At the high power end of the scale, I would personally find valves hard to justify, but again some people swear by huge transmitter triodes carrying over 1KV on their anodes, and they can be truly awesome sounding, but the expense is frightening!
I find it hard to articulate the differences between valves and transistors, and the experience of both hearing and of building your amps, has made it more difficult still. NVA, Nelson Pass, and Nick's amps don't sound like the stereotypical fizzy treble afflicted amplifiers, us valve lovers, love to hate, so in the silky top end stakes, there is little difference with these manufacturers.
There is a certain warmth and humanity about the presentation of some push-pull pentode and ultralinear valve amps that is beguiling and is a selling point for a lot of people, especially those who were burned by the flat earth bullshit. And having built those kinds of amp, and having myself been severely burned by the aforementioned bullshit, I can see perfectly well where they are coming from.
Low power SETs can be built with more of that warmth, extended down to the bass, if your power supply is inadequate and the driver circuitry is less than great, but a top SET, with a proper power supply and good output transformers has none of that artifice. Nick's 300B amps driven by a 5687 cascade and with his trademark top power supplies are a case in point. The bass performance into efficient speakers is as good as a transistor amp and the rest of the frequency range is clear as a bell. I'd also like to think that my own 2A3 SET with its big power supply and power valve drivers to the output stage, is also like that, though the output transformers could stand an upgrade.
I think, with valves, the interface with the speakers is a bit more of a challenge, but someone other than me would have to explain why and how, as I have bit of knowledge, but not enough to be authoritative.