I'm coming to this a little late but I'll throw in my penny.
Just so you know where I'm coming from...I've practiced Seidokan aikido for nine years now. It's the only aikido style that I've actively studied, I've only been to a couple of seminars of other styles.
I'm not going to tell you that what we do is the best thing going and that it holds everything you need or want.
I'll echo those who've said to visit the various dojo available. I think it's far less important to pick a style than to pick a sensei you feel you can learn from. My sensei happened to teach Seidokan, if he taught something else I'd be doing that.
Like most aikido out there it really is difficult to classify Seidokan (or any styles) technique. You'll find teachers who only want to focus on ki development, others are only concerned with martial application, others want precise technique, and some care less about what external form looks like and more that the principles are followed. Most people will probably be a mix off them all. Again find a teacher you like.
Some technical aspects you'll probably find/not find in Seidokan are:
- The previously mentioned version of kotegaeshi. While not exclusive to Seidokan this is our standard version instead of being a variation.
- No head controls
- No koshinage (hip throws)
- Few techniques that require breakfalls (high falls)
- Little to no atemi (striking vital points)
- Our standard shihonage is a controling technique instead of a projection (throw)
- Strong emphasis on teaching. Usually start teaching/assisting relatively early in training.
* These are, of course, dependant on the instructor. While these things (and others like ground techniques) are not part of the "official" syllabus some instructors may still teach them.
As far as differences from other aikido I'll leave that for people with more experience with other aikido
That's just my take. If you need me to elaborate on anything let me know....you may want to email me through the AikiWeb system as my school responsibilities often keep me away from the forums.