For DD enfamil because that was what our pedi gave us samples of, and they were constantly sending coupons which I could use with BJ's coupons to make it cheaper than generic brand. After about 8 months I switched her to generic BJs brand. I loved BJs, they were always sending store baby coupons that could be combined with manufacturers coupons -- when DD was 11 months old we moved out of state, no more BJs :-(
For DS we used Similac because I had free samples and tons of coupons through my OB and the kids' pedi. After about 4-6 months I switched to Target brand to save money.
This LO will probably use Enfamil because I already have a ton of samples. Both other kids didn't have any issues and need to switch, but some babies it takes a while to figure out what will work for them in a formula. Target brand is pretty good, but clumps more easily than Similac. Enfamil clumps more than Similac also. At least in my experience.
I'm on amazon moms too, but don't feel as though their formula prices are that great. I hate BRU, but really like the concept of the VIB card because you are getting 10% off diapers, wipes, and formula (as well as baby food down the line), this can be combined with store promotions and coupons as well, so if you are buying name brand, it's a pretty good deal. For example, I loaded $100 on my VIB card, automatically got $10 back on the card for future purchase; bought 2 boxes of pampers ($47 ea) and a value box of wipes ($15), used a manufacturers coupon for $8 off, got a $25 gift card plus $5 gift card for an in store promotion... so basically $61 for $109 worth of diapers and wipes. Similar deals on formula. HTH