Re: If you have trained in both Aikikai and Iwama, please help me...
I originally trained in Hombu style Aikikai for 2 years, but since having moved location I was forced to look for another dojo. The only Aikikai affiliated dojo near me was Iwama ryu so I started going there and have been training there for almost three years now.
The main difference I found is the comparitive lack of awase training in Iwama style compared to hombu. Although we train in awase, the emphasis is much more on hard techniques - as in from a strong grab or stationary attack. The reason behind this is too strengthen your basics thoroughly enough to allow a natural development in speed and intensity.
I also found that there was a surprising lack of similarity in the techniques of the same name. Of course the basic principles are the same, but the motions differ considerably. There is of course a much larger emphasis on weapons in Iwama than in Hombu.
Basically I can only speak for my dojos, but the pros and cons as I see them are:
In iwama style I was pleased that the training was very particular about accuaracy - very picky about every body movement, so that you really have to make sure everything is in the right place. Also the weapons training is great and the fact that you train from a really hard grab or solid attack means you can actually learn to move an unwilling uke despite their best efforts - oh and there is a lot of atemi in Iwama style.
In Hombu style, my dojo was very geared towards free flowing and natural techniques, a lot smoother than Iwama style. Some of the techniques that I was fond of such as Hanmi hantachi suwari waza shiho nage (ura) don't appear in the iwama syllabus which I think is a shame. I also really enjoyed the strong emphasis on ukemi in Hombu. In iwama, you are not "taught" to take ukemi, but are expected to develop it yourself from taking techniques. Some of the folk in my dojo didn't particulary look forward to some of the throws.
I can only speak from my experience in 2 dojos, so if I have misrepresented these styles then apologies. I actually would like to train in both, as well as visiting a Ki society dojo sometime as well.
Hope this helps