Among the tendencies prevalent in aikido today are the soft approach emphasizing circular or ki no nagare techniques of Doshu Kisshomaru Ueshiba of the Aikikai Hombu Dojo, the so-called hard-style school of Yoshinkan Aikido headed by Gozo Shioda Sensei, the emphasis on the concept of "ki" of Shinshin Toitsu Aikido as espoused by Koichi Tohei Sensei, the eclectic system of Minoru Mochizuki Sensei of Yoseikan Aikido, and the sports aikido system which includes competition devised by Kenji Tomiki Shihan. To these must be added the unified technical curriculum formulated by 9th dan Aikikai Shihan Morihiro Saito. Saito Sensei's approach stressing the interrelationship between taijutsu and bukiwaza
by Stanley Pranin
I thought this might be useful to you as well.
As for my thoughts on the different styles of Aikido, I think there are millions, as every has his own way to do techniques, even though they are similar. It can be likened to reaching the summit of a mountain there is an infinite number of ways to reach it, we all just take different paths, no one is wrong but the one that is probably most right is the one Osensei took.
Yes it can be said that some are hard and some are soft, but there are also hard and easy ways to climb a mountain. Different people trained with Osensei at different times along his path so when he was at a 'hard' point they learnt a harder way.
The one thing I have noticed so far that is great about Aikido is that no one claims that they are right and every one else is wrong, I could be wrong but we are trying to follow Osensei to the top of our mountain.
If you want more specifics then just ask?