Treasury bill pays more at the moment it gives you 13.9% for 90days, 17% for 182 days and 18.2% for 365 days tenor.
Mind you treasury bill pays interest up front. For instance 10million investment for 90 days will give you almost 400k. This you get on the day of the bid and your 10million is returned at the end of the 90days. It's cool money when you have plenty money to invest. Either in the primary or secondary market.
Fixed deposit rate although has a bank rate is largely determined by the branch you entered. The branch will always give you a lower rate because the lesser the interest to you, the more money the branch makes.
The best rate any Nigerian bank will give you on fixed deposit is 12% and that's if you are bringing up to 50million.
Go for treasury bill