If you are looking at the snugride vs snugride 35, I suggest the larger one because it lasts longer. There are babies who outgrow the little snugrides by 3 months! My 13 month old still fits in his larger one and isn't even on the highest strap position yet and he is a tall baby. When they get too big to carry you can just leave it in the car all the time, put off buying the next seat and you get that option that if they fall asleep you just leave them in the carrier and carry the whole thing with you.
When you are buying it, check the manufacturing date and try to get a newer one. While you can get a good sale on occasion it is often because that seat is already a year or two old and car seats do expire.
Usually your best bet is to find a coupon. Babies r us has a 20% coupon out in the mail that started yesterday. They also give you coupons at check out. You could always ask at customer service or go through the line and buy a candy bar or something to get the coupons they will give out. (they are different from the ones in the mail)
If you can find a buy buy baby store, they take both their own coupons and the bed bath and beyond ones. You can even usually get them to take expired ones, so that is an automatic 20% option they always have.