I've heard both, in similar systems, though not in any sort of side-by-side comparison. The ProAc is somewhat of a rare beast--it actually uses a ribbon tweeter that is well integrated with the other drivers. In some respects it is a "safer" choice in that it will always sound good. I like the Kensington for its fast, reasonably dynamic sound. But, to me there is a peak in the upper midrange-treble region that adds a touch of sibilance or hardness that I find slightly annoying (others seem to think it makes the sound crisper and more exciting, so it is a matter of taste). While the speaker has driver level controls, these do not remove the peak entirely without otherwise hurting the sound.
Both speakers play reasonably well with medium powered tube gear, which is a minimum requirement for my own preferences. Because I did not actually compare the two, I will not hazard a guess as to what would be my preference, so I will stick with saying that I liked both speakers.