I registered at Amazon & BBB, and then did a "secret" registry (not publicly visible) at BRU & Target just to get the extra completion discounts. I was also able to find a coupon for a $20 gift card at BRU when you start a registry, so I got a free gift card plus the completion coupon.
All of the stores offered a gift bag of freebies (mostly coupons, some free bottles/onesie/pacifiers) for registering in person. Supposedly you can go in to the stores and request your bag if you do it online, but I think the gift card one we had to start in-person. What we did was to sign up in store, scan a few basic items (diapers, wipes) and then go home & do everything else online.
I liked BBB better, because their coupon policy is much better. On inexpensive items, I could get things much cheaper using the $5 off/$15 coupons, plus they also let me pair up manufacturer coupons on top (for bottles, pacifiers, etc). I liked being able to use the 20% off coupons on higher priced items that man stores don't carry, like my $50 Bravado nursing bras.
Some of my friends don't have BBB anywhere near them, so BRU would have been easier for those that like to shop in-store. If you want Tommee Tippee, BBB does not carry them - so go with BRU or Target.
Target overall has lower prices, but a limited selection in stores. If your friends are savvy shoppers, the big box baby stores have some great deals with coupons - but if they don't have/use coupons, they'll pay more for the same item.