Server / waitress or bank teller?

Do both. Neither is a full time job and I'm sure you can work out a schedule were the two jobs don't conflict with each other.

Server/waitress advantages: Tips, more opportunities for higher pay than your salary.
Server/waitress disadvantages: Irregular hours, not enough hours, no tips on some days, harder working conditions.

Bank teller advantages: Inside job with regular hours, may be promotable after a while, possible opportunities for moving up and more pay
Bank teller disadvantages: highly secure job--detail oriented work, little tolerance for mistakes or errors.

You need customer service skills for both--more at the server job because more people will act like jerks in a restaurant than in a bank. Can be frustrating when days are slow and you aren't making tips. You may have to come in at odd times or on the spur of the moment at the server job, you won't at the bank. You need to like people for both jobs--and have patience and tact. Good people skills are key at both jobs. There may be a dress code at the bank, and you may have a uniform you need to wear at the restaurant--both will be fairly conservative in dress.

It depends on what you like best--regular work or somewhat irregular work--you may make more at the restaurant, but you'll have a little more security at the bank; restaurant work is notorious for high turnover. Banks not so much. Both places CAN get a bit overwhelming during peak hours--and you'll need good detail skills for both to keep you on track.