Help Needed: Moving from Corporate Banking to Investment Banking

Hi All, Really need help making a right decision here. I’m a first year analyst in a corporate banking division at small European bank, trying very hard to move into IB. This week I received an offer for a big Japanese bank with the better pay and group, but also within corporate banking. Yesterday had a second interview with a MM Investment Bank, and they asked me to let them know an expiration date of my offer so we can discuss the next step. I have two more days before expiration and I am not sure what I should tell the MM bank. I never reneged an offer before and I am very nerveous on making a decision here, but IB is something I really want to do.

FYI: MM firm is very fast growing, they are very acquisitive and has tons of opportunities to learn and grow.