I decided to try these as I previously had a complete set of Ventus Prime 2 K115 and the promise of being a little more sporting was appealing. Both the 2s and 3s are good tires IMO when it is remembered they are touring tires and not track biased in any way.
The first thing anyone will notice is noise and comfort. Surprisingly, the 3s are actually a set backwards. The 2s are noticeably quieter and round off road imperfections more effectively. In this department, both are superior to the heavy Conti Premium Contact 2s. The Kook 2s are almost as good as the lightweight Pirelli P7 Cinturatos with regard to comfort which on my other 156.
Next is response and feedback. The Ventus Prime 3s are a small improvement over the 2s in this respect. The 2s are obviously aimed more for comfort. The 3s are similar to the Pirellis but not as good as the Contis which need less lock to get round bends. The 2s are adequate but not inspiring.
Handling. I thought I'd have more to say given the crazy angles I've put the 156s round bends but they appear to be safe and manageable. Perhaps that is in itself great praise. Contrast that with the Contis which become messy, inaccurate and totally lack adjustability and either of the Kooks really do shape up to look controllable in extremis. They are perhaps not as good as the Pirellis though. The only criticism I have for the 3s is that cold weather does drop off a little (as it does with all summer tires). In this area, for once, the Contis make sense. However, the good handling means it is easily manageable and the reduction is in no way concerning.
Wear. I'd say the 3s have it. I can't be sure but now the 3s have less than 3mm and over 20k miles (a first for me) I think the 3s are a particularly durable tire. My impressions of the 2s wear I though wear was on the high side but I didn't mind. I did mind having to rub loads of tire black into the tiny sidewall ribs of the 2s though... Another thing about these tires, notably the 3s is how well they preserve the grip and handling characteristics when they are worn. Budget tires cannot do this and become terrible (a set of Vredestein Sportrac 3s became decidedly psychotic- yet were winning tire tests). They may not be up to Michelin Pilot levels in this respect but they do perform convincingly even when worn so can go right to the 1.6mm.
In short, either variant is recommended. The 2s are quieter and more comfortably and the 3s feel a little more sporting even though there is not much difference in outright grip.