Ever get stuck trying to change strings? In response to a customer's request, herewith is a handy-dandy how-to guide to string changing.
In mid 2001, D'Addario brought out their "EXP" series ("Extended Play") which have a chemical coating of the windings on the strings, rather than a film enveloping the whole string. These have a fuller, warmer sound than Elixirs, and seem also to have a very long life.
In September, 2001, Martin released their SP+ Extended Life strings. These have a more natural feel to me than either the Elixirs or the D'Addarios, and hold up quite well. Frankly, I'd give the longevity edge to D'Addario, but sound and feel to Martin.