Torn by giving information on this situation, it seems to me the best thing to do is reconsider your perspective. You are a veteran, your sensei is accommodating, and it sounds you sensei has a strong conviction. Don't you think than this is a matter of differing opinions? Your sensei has been at Aikido longer than you and has seen more of Aikido. He or she may have a good point, a point you haven't come to see.
As for your other problem, don't play with fire if you don't expect to be burned. A hugely important lesson, I learned when I was dating. Any good frat or like group do expect a level of loyalty and respect, and you been with your group for 15 years. Do you think it is unreasonable your group would, including the sensei would be upset that you trained with a rival group? In the work place that is a cause for losing your job. In at a higher level of organization like the government and military that is treason. Would it be bad to suggest some personal reflection and personal inventory to assess what you really want?
Broaden your scope, take a more dynamic attitude, it may help you make the right decision. A decision which could lead you to harmony and happiness. Good luck!
Thank you for your opinion. I definitely understand your perspective and views. I in my own view I do not see this other group as a "rival". I try not to view any other groups as rivals but as other ways up the mountain. I think what I would like is a broader perspective and true harmonious view of Aikido rather than us against them. I am loyal to my Sensei and our group but I think my path and views are diverging in attitude and training. I am really bothered by this internally.