As a mainly defensive player I tend to favour the defensive type units. So if I'm playing as catholic I do dismount my chivalric knights often. Basically you can look at it quite simply: If your enemy is attacking you with chivalric knights, dismount yours, you'll win every time... well thats what happens for me anyway...

I find that in a defensive situation dismountable heavy cavalry are best dismounted. I've even dismounted my 20 man royal bodyguards the same and watched them see off everything on a horse that attacked them. Works wonders in woods or with a height advantage. The woods in Flanders are perfect though.