I would not using anything with bleach - it's worse than the ring, for the environment, for your breathing while you're using it, and for the skin of the next person who sits in the tub (no matter how much you rinse). Sometimes "clean" is a lot more harmful than a little dirt! A lot of the cleaners emit fumes and just aren't healthy to breathe. I don't think paper towels hold up at all, and they are a waste of money and trees. I just saw a report on Good Morning America about how a lot of the "green" products are misleading - they may have seals that state they are "better for the environment" or "all natural" but often they aren't certified by an independent organization - the manufacturer just sticks a green leaf on the label!
I'd use baking soda unless you have a tub surface that will scratch easily. I use a scrubby and make a paste of the baking soda. If you do it often before the ring gets bad, it's easier all around. It rinses cleanly and thoroughly, and you can use it on the shower curtain too. It's cheap, safe, and effective.
Good luck!