TNT,
the core wire is tin plated for both nickel plated steel strings and pure nickel strings. The tonal difference primarily comes from the wrap wire.
Both the GHS Boomers and the Ernie Ball Super Slinky strings use nickel plated steel as the outer shell of the strings. The main difference between the two is that the Boomers use an alloy core that the shell is wrapped around, as opposed to the steel core that the Super Slinky's use.
GHS Boomers Versus Ernie Ball Super Slinky Electric Guitar Strings - Associated Content - associatedcontent.com