There are three ways of making payments.
2 Years: all at once. Maximum discount, lowest per price month. Promo codes at full value.
1 Year: all at once. No extra discount but it does eliminate the setup fee. Promo codes at full value.
Monthly: Pay every month - limits the value of most promo codes (something like $50) AND you pay the setup fee to start
You need to pay with credit card or Google Checkout to preserve your 97 day money back guarantee.
Note - if you register a domain at signup, this will NOT be refunded for any reason.
I believe this is pretty correct and hope it helps you decide. Almost every signs up for 1 year as it's really the cheapest way in, but you get a decent discount per month at the 2 year if you can swing it. Remember you have 97 days to back out if it isn't for you. Upgrades are VERY simple and you get full credit on your unused balance - UNLESS you go from yearly to monthly - then you pay the setup fee.