Hi Hocky,
ML recommends 200 wpc as a general rule, and I find it holds fairly well. I also generally fall into more power is better category. However, there are many other factors in play as well, such as amps design, quality of parts used, and power supply design that can make a lesser powered amp sound better than a high powered amp.
Case and point is the Sanders amp. I gave it a 30 day trial and found it very good for its price point. However, although Sanders is rated at 400 wpc, I found the Ayre V1 - Xe much more to my liking. I played the Sanders for a week. Once I plugged the Ayre in, I did not plug the Sanders in until the last day I had to send it back, and I rushed off to the post office to send it back. Charlie Hansen of Ayre made Roger Sanders look like an amateur. The Ayre is only 200 wpc, but costs twice more htan the Sanders.
At a similar price point as the Ayre, there is the CJ Premier 350. Once I plugged that in, out went the Ayre...Conrad and Johson made Charlie Hansen of Ayre look like an amateur...So in the case of similar price points, more power is better...
Now I think the CJ 350 is as good as it gets. However, I am always exploring. I put a couple of Atmasphere monoblocks on my Spires out of curiousity. These guys are only 140 wpc, but cost over $20K. They smoked my CJ 350 in every area other than bass articulation and a little extension of the highs.
So, yes, power is important, but design and parts are important as well. Of course, there is also personal taste. Many guys like a low powered tube amps, and there is nothing wrong with that either...