formula brand (supplement)

Amber
We have decided to supplement our breastfed baby, who is just about 5 months. We started about a month ago due to pumping and stress compounding with a baby who loves to eat! 
My question is... if you are on a formula that works well for you, do you switch to a different brand if you get coupons or see it in sale? 
Right now we do one formula bottle a day using similac for supplement (green). But we keep getting coupons for enfimil