Don't forget the 88-form, that is in fact the long form, more or less unaltered.
Marc, I am not familiar with this form or the Li Family's 81-from. Is the 88-form symmetrical?
This discussion has made me wonder what I will be missing if I do not go on to the long form.
One thing I forgot to say is that I do wonder whether simply doing one repetition of a short form is sufficient practice for those who really want to study the art in depth. I think that some Tai Chi is much better than no Tai Chi, but 20 minutes of Tai chi is probably more than twice as good as doing only 10 minutes. If your typical home practice is just one repetition of a short form, this might not be the best foundation to go deeply into the art.
You can do a great deal with even just one posture, let alone 24 or more; however, I personally do not know anyone who seems to have made great progress using predominantly short practice sessions. The issue is not the length or the character of the form, but the length of the practice. Consistently practicing only one repetition of a fast long form would raise the same concerns for me as one repetition of a short form. If time and interest permit, we actually encourage the goal of doing three repetitions in a row (each of 20-30 minutes) to get a great benefit, but I know few people who can manage this kind of time commitment.
I think also that the better forms have a very deep logic to them, sometimes developed over several generations. Most of the features of these forms are quite deliberate and should not easily be dismissed as unnecessary, ineffective, or insufficient. This does not mean, however, that there is only one valid way to train or that everyone should like every flavor of Tai Chi.
Lastly, I think it seems to have been fairly common for practitioners who start off practicing short forms to decide at some later time to seek out long forms in order to deepen their practice. I think only a minority of advanced practitioners encourage doing only short forms for their entire careers.
I still practice it almost everyday both styles of walking.
Do you prefer doing the walking to doing the form, or do you do it in addition to the form? Does it include hand movements, or just footwork?