What determines good distortion?

This is a really old question.
SS amps tend to clip 'hard'. They really have a flat top to the clip waveform, and thus create really strong high frequency output when clipping. (what that is, and means to a layman is too complicated for me to enlighten you, get a book on amplifier disign)
A tube amp clipping creates rather 'soft' rounded clips,(way less high frequency junk being created) and thus are gentler on speakers.
So, in general, the tube clipping is way more survivable, and rather 'easy' on both speakers and ears.
The SS amp will clip and sound grating and harsh, besides killing your tweeters.

So neither of you are 'wrong' You just have different experiences.
Yes clipping is bad. Yes a tube amp will be able to clip and be less problematic for both hearing, and tweeters.
So you are both right.
And he may just like the sound of tubes... which is his right in general.. Then you have those who say 50 tube watts are 'bigger' than 50 SS watts...

But I bet neither of you usually run the amp so hard they produce clipping anyway. So why worry?

Some solid state amps have clip protection. Such amps produce a waveform smoothly limited in the same way as a tube amp. Also, mine is good for 600 watts, so I don't think I need to worry.

What I've always heard is that SS produces odd order harmonic distortions (3rd,5th, etc) while tubes produce even order distortions (2nd, 4th, etc). I've also heard that musical instruments produce even order harmonic distortions, which is why most folks find tube equipment more "musical" or "natural", because it's distortions are more similar to that heard from a musical instrument.

J, I think you are mistaking tube for single ended and ss for ultralinear. An old Pass Aleph does not distroy even order harmonics like an old ARC push pull.

You will hear a lot about ss sound and distortions vs tube sound and distortions. First, eliminate the stupid. A well designed and built amp will be wonderful, regardless of whether it is tube or ss. As Jmcgrogan2 stated, ss designs "typically" produce odd order harmonic distortions, where tubes produce even order distortions. This is only true so far as how the amps were designed. circuit designs can correct for distortion effects. it totally depends on how much effort and money the designers placed into their designs. So, please do not buy into the ss sound one way and tubes sound another way. you can have a tube design that is basically crappy circuit design, same as with ss. So, it is a combination of the actual circuit designs (regardless of whether it is tube or ss) and then you add in whether they "corrected" for particular distortion effects in the design. I have heard (within specific price points) ss amps that were much better than tube amps and visa versa (within the price points). At the upper end of the price range, i.e. rediculously priced amps, I would take either one. The ss amps sounded wonderful and so did the tube amps. One is definitely not better than the other. It depends on price, circuit designs and other factors. Please don't buy into the tube is better than solid state or visa versa arguments. Pick a price point, then go demo in your home, using your equipment the various amps and see/hear for yourself. depending on the amps, you will probably pick a ss amp and in other cases, the tube amp will make you smile.

stay away from the hype.

enjoy