No offense to anyone, but I feel aikido koshi nages allow uke to fall; judo koshi waza propels uke. I think this is because the judo applies more rotation in the hips during the throw while aikido kinda throws the hip into uke as an obstacle. I also think judo throws place far more emphasis on "fitting", whereas aikido people are less precise (possibly because of compliant partners). Aikido and judo also have two completely different uke responses. I bring this up since I think we need to distinguish between aikido and judo in describing our preferences.
I think Jon has a limited experience of Aikido's koshi nage. There's no practical difference between the Judo and the Aikido version of it when it's executed properly. I've experienced the propulsion he refers to in both arts -- and I think it comes easier to Judoka because they practice it as a core of their art.
Aikido's koshinage has always been a challenge, both for uke and for nage, because of the levels of commitment, confidence and height involved. Most of the koshinage 'practice forms' I have experienced are designed to get uke and nage in proper alignment and give both a safe starting point to experience the technique and it's results from. This requires the three P's (practice, practice, practice) and a heightened awareness of risks. No one wants to injure themselves or their partner by improper training. Getting to the 'art' of it from means finding a safe way to practice the 'static' forms, so the open and closed stance variations both have their places -- but don't confuse them with a more dynamic experience of koshinage.
Koshinage is a dynamic and beautiful redirection of energy that doesn't really involve "loading up" or "removing a supporting leg," because the transition from attack to throw doesn't have a real stopping point in the middle where one should have to support uke's weight, and does often involve propelling the uke further upward and outward as a result of the throw. The most effective applications of the technique, as witnessed when it appears in randori, happen in a manner that is not easy to duplicate in a static form, simply because neither partner ceases to be moving -- nage is not "planted" nor is uke "just holding on". When we practice to learn koshinage, we need to "stop time" to ensure both partners are in approximately the proper alignment during the execution of the technique -- which is why we "load up", I think -- but this is not a practical application of the martial art, just a training tool to acquire the skills to execute it and take the resulting ukemi.
Train safely, and don't confuse the 'training'
with the 'art'
of the technique, is I guess what I'm trying to say.