The longer the scope, the more extraneous noise problems you have. So, the shorter the better. Also keep in mind your willingness to lean over the bed. If you routinely examine patients from five feet away, then go for the longer one. If you're up close to the patient when you examine him/her, then the shorter should do you better.
I have heard that the 22" ones are too awkward to deal with when seeing patients. Do a search on it--there have been a ton of stethoscope threads in the past. That's what I did when I had to buy one a few months ago.
According to the reps who sold the scopes to students at my school, the long scopes do not have this problem anymore. They say its a thing of the past and has been this way for quite some time. You can't go wrong with the long scope. Plus, the long and the short scopes are the same price.
Unless they have changed in the last 2 years, then I can honestly say that I speak from experience -- the longer scopes have more extraneous noise (but it is tolerable). I guess for someone at the bedside in a hospital it's ok. For someone in a moving ambulance, it's not ok.