I own both KEF 104.2's and Thiel 3.5's. And, like someone
already mentioned, they are two different animals.
Speakers tend to reflect the taste of the designer. Although my Thiels are an older model, most all Thiels are
cut from the same cloth. Their balance is definitely tilted
upward. Jim T. likes em on the brighter side, so careful
system matching is important. When matched properly, they can be very detailed and sweet, but if your electronics are
even a little on the bright side, you'll find yourself running for the volume knob more often than not.
The KEF's, on the other hand, are a more evenly balanced
speaker and tend to be less fatiguing. They are also more
dynamic and far more efficient at 92db than the Thiels.
They can be driven by as little as a 15W SET amp and still play beautifully.
It's a tough call, you picked two excellent speakers, but
if i had to choose, i'd probably lean a little toward the
KEF's for long term listening.