Likely the areas where you will have the most need for additional power is in large orchestral classical pieces. This is due to the wide dynamic range of such pieces. But the difference between a 140 watt amp and a 200 watt amp is not going to matter a whole lot. For example. when I listen to a Beethoven symphony, the sound peaks of about 105 db spl draw right at 400 watts from a mcintosh amp. However, a good portion of the non peaks draw less than 40 watts and the quieter parts are in the 0.4 to 4 watt range. I state this a an indication that the benefits of the extra power come in more when there is an order of magnitude or more difference between what is available. Does one really need that much power? Not in my opinion, I just like it. My own bottom line for what its worth, which is based only on my subjective philosophy, is so long as you have enough power to be able to hear the quietest part of a work, and, without changing the volume setting, the loudest (or most power hungry - which will be a lot of bass) part of that work does not result in clipping, you have enough power to appreciate the music. The best test I can think of offhand is Mahler's 2nd.
When you get away from big orchesteral works and particularly if you are listening to music without a lot of bass, for example a piano sonata, your power requirements are drastically reduced.
One solution to get some of the best of both worlds, is pick up a pair of decent headphones for the occasions where you want to listen to something at higher volumes than your amp may be able to provide.
In sum, I would not make the choice of the two amps you mentioned based on the difference in their power specs but rather on how they sound to you at listening levels that they support.
Remember, also, that some opinions as to what is desirable are extreme, especially where it comes to power and loudness. In a perfect world I'd have an amp and speakers that could go to 140 db. - problem is that I know me and I know that all I'd be hearing inside of a year is ringing due to tinnitus from acoustic trauma.
