To Diego Estan,
In this case, I’d argue there is no “better” speaker, because they are both very good. However, they sound quite different.
Both are excellent in the bass department for speakers of their size, but in my room (confirmed by measurements), the PSBs had a bit more output and extension—so much so that they almost redefined what I thought was possible from a 6.5″ midrange-woofer in a two-way design in terms of bass!
In the midrange, I’d give the edge to the 705 S2 in terms of transparency and detail retrieval; but again, only just.
The treble is where you’ll find the biggest difference—between 5kHz and 10kHz, there’s an extra 5–8dB out of the 705 S2 tweeter. This is absolutely unmistakable and immediately noticeable. The PSB is voiced to sound neutral, while the B&W is voiced to sound exciting and attention-grabbing, with lots of “air.”
Which speaker is better for you will come down to personal preference and listening habits—mainly because of that difference in the treble. If you like to listen to your music fairly loud (say 90dB or more at the listening position), go for a pair of B600s. You’ll likely find the 705 S2s will sound fatiguing—and for some recordings, even irritating—over time. If you’re a low- or medium-volume listener (say 70–80dB SPL), you may prefer the extra air and detail retrieval that a pair of B&Ws can provide at these levels.
The speakers are around the same price, so my recommendation is to listen to both if you can.