If your briefcase fits under the seat and is tight in the bin, I'd recommend putting it under the seat. On a recent flight a PAX forced in a smallish roller bag making it difficult to open the bin. He closed it, but then the FA came through checking for space and when she couldn't open it fully without breaking the bin, she called maintenance and this delayed the flight a bit as they wouldn't leave until the maintenance got the bin open and made sure it was not broken and removed the luggage, gate checking it. The PAX involved were totally unapologetic to the rest of the PAX or the FA for causing the trouble and were a bit upset when the FA told them she was gate checking the bag, wanting to know how they would get it back. They seemed to think it was perfectly ok for them to try to force the bag in the bin and outrageous that the FA even tried to open it to check on it.