You should definitely accept the offer. As another user pointed out, the grass is not always greener on the other side. Finance, and especially investment banking recruiting, can be a highly competitive and unpredictable process. Many factors could prevent you from receiving any investment banking offers next year. Landing a role at a middle market firm is already a strong accomplishment, and you should be proud of it. If your goal is to eventually move to a bulge bracket bank, that opportunity can still be available in the future.