I think it depends on what you have wrong with your knees. Mine are perfectly functional for non/low-impact things (so cycling is fine, walking is OK for anything up to a couple-three miles, running is Right Out), so both boarding & skiing are fine for that. The problem is that they're not very resilient to being turned in awkward directions - on such occasions, loud crunching noises and pain happen, and the pain tends to continue for a while afterwards. This has in the past happened when standing up or turning awkwardly, although it usually has to be a bit more dramatic.

Anyway: with skiing, when you fall over, your knees have a tendency to go in opposite directions, and often directions in which knees would prefer not to be. Hence, loud crunching noises, pain. And, consequently, a tendency for me to be a bit under-confident about attempting things, because I know what the consequences are if I go splat. I think it's true of a lot of physical things, but certainly my experience of skiing (& it's also true for boarding from what I've heard) is that confidence is a major factor in being able to do stuff successfully. With boarding, your knees are strapped nice & securely in the *right* direction, so falling over, whilst bruise-inducing, is not knee-crunching. At least, this is the theory - I certainly know other folk who've switched to boarding for similar reasons. Hence, we hope, more confidence, and thus more fun.