Yo.
Tried and tested route for getting a longer lasting string; buy a good string, like a set of EBs, and after having put em on properly (which I'm sure you do) always wipe em down with a duster before your sweat dries on em atfer your gig/rehearsal or whatever.
A wiped down string will always last longer than a non-wiped one.
And don't use fast fret and stuff like that - any spray or oil you add to a string will take away from it's lifespan.
As it happens a nickel string tends to be less bright than a steel string when new but dies off a lot more slowly. Steels will seem more metallic when new but will drop off more quickly. Try wiping some nickels?
The heavier the guage the better a string lasts too.