Maybe you should also invest in an on-board battery trickle charger. 2 miles daily or a lot of short trips isn't going to fully recharge your battery. The biggest battery drain will be from the engine startup, not to mention some car manufacturers tend to test the O2 sensors when the car isn't in use (turned off)...so there is a slight battery drain whether one knows it or not. Just plug the trickle charger in once a week (or more often, esp during the winter)...overnight or longer...to fully recharge it. Here's an example of one:
http://www.shop.com/op/~SCHUMACHER_E...0?sourceid=298
You can find the same model/design sold under different brand names. Walmart used to sell them under their Everstart line. The cost of the on-board trickle charger is about the same as a longer battery warranty and the small size means it can be used in about any application. Just run an extension cord to it and plug it in and close the hood or run the cord under the car up to the battery charger.
Paying extra money for a longer battery warranty is just that...buying/paying for the extra coverage. You have to decide whether the warranty is worth it or not, especially if the battery specs are the same. Check the size of your battery tray. You could buy a larger battery with more capacity...just have to cross check the length, height, and width dimensions with your current battery, some OEM battery trays can accommendate a longer length battery, but are height and width are restricted due to clearances under the hood. That's where the Optima, Odyssey, and Exide Orbital come into play since you can place those batteries on their sides as well as traditional postions and not worry about leaking.