Hello, i was wondering which weapon training would best fit Aikido. I practice Iwama Style, so about 1/3 of our training is with the ken or the jo. I love it, so i tried to underestand from where did O sensei derived it, and it's a bit confusing. I understood that he learned some Kashima Shinto Ryu (right?), and many shihan (e.g. Tissier) study Kashima s.r., but often i found Aikido sensei that practice different koryu. For instance, Yashuhiro Saito, current Iwama (Takemusu) aikido leader, holds a menkyo kaiden in Katori Shinto Ryu, while at Katori S.R. homu dojo there are aikido classes.. why is it so? wouldn't be more logic to link aikido with aikiken "granfather" Kashima S.R.? Technically speaking, what are the biggest differences (kamae, distance, timing, striking, strategies etc.) between these two main koryu?
You've asked a rather complicated question with a fair amount of incorrect information.
O-sensei observed (not trained) for two years, the basic kenjutsu of Kashima Shinto-ryu, taught on a weekly basis to his son, and one uchi-deshi. Using the sequence of movements, he "plugged in" his aiki interpretations of swordsmanship, further interpreted by Saito Morihiro. It is only the first one or two of the Iwama kumitachi that correspond in movement sequences to that of Kashima Shinto-ryu, but overall, they are profoundly different.
Starting with Inaba Minoru and Yamaguchi Seigo, the latter whom Tissier followed, they studied Kashima Shin-ryu. Although the names are similar, they are about as different as the freestyle and backstroke in swimming. To complicate things further, what Inaba, et al are doing is, although excellent in its own right, not classical Kashima Shin-ryu. According to practitioners, this Kashima no Ken is very different. [easy to see by comparing youtube videos]
Finally, a smaller issue - Saito Yasuhiro has a mokuroku in TSKSR (I don't know what line) - not a menkyo kaiden (unless his rank was "increased" since any online comment was made).
As to the answer to your question: Kashima Shin-ryu would conflict radically with what you do in Iwama weaponry. Kashima Shinto-ryu is not international. If there are any licensed instructors teaching outside Japan, it's news to me. You'd most likely have to learn it in Japan.
If your teacher is Saito Yasuhiro, then it is likely that his version of Iwama weaponry is now influenced, to some degree by TSKSR.
And here's one more point: You ask: what are "the biggest differences (kamae, distance, timing, striking, strategies etc.) This is simple. Neither of these ryuha use the same kamae, the same ma-ai, the same timing, the same striking or the same strategies).