The better one ($0.99) only works for customers located in the US or Canada, while the $2.49 coupon should work for everyone. Both of them only work for one domain and if I’m not mistaken, you can only use them for dot coms (so for a dot com new registration or transfer).

First of all, here’s the $0.99 coupon:

99COMS

And here’s the $2.49 discount code:

CAG249