D'addario Slowounds. Not super bright, but the tone out of the package lasts for months. Very consistent, too, along with GREAT feel. Nickel plated, colored windings, about $28 for a 5 string set. Mine usually last about 3-4 months...and I kill steel strings in two weeks.

Great stuff.
As far as which strings last longer...nickel lasts longer than steel strings. They're less bright, however, so if you like the brightness, you have to buy steel. But, you'll end up replacing dead strings far more often.