As Phil says, technically a large difference in conductivity.
But there several very interesting aspects to power delivery, and the most obvious fact is one of the least concerned.
First, as in any rail, the cross section is so large that the voltage drop in rail alone is minimal, it's way bigger than any wire in your house typically. (although I have some 6 gauge)... so brass vs. stainless steel, vs aluminum is not really the largest factor in voltage drop/loss from your power supply to the loco.
As with all rail, the resistance (read voltage loss) is from connections, the rail joiners and the attachment of feeders, and the rails to the wheels.
For all intents and purposes, SS does not corrode or oxidize, it never needs oxide removal like brass or nickel silver (note well nickel plate is not nickel silver).
So conductivity is always great, hitting the track with a hose to wash off dust or lawn clippings is all I do, and wipe with a cloth for the "black stuff" that forms on all powered rails.
So salt, pollution, etc. does not affect it. I can leave my layout for 6 months, hose it off or a quick 5 minute wipe and it is perfect.
Best decision I ever made since I determined I was going track power and DCC.
There is one other benefit specifically for DCC, the DCC signal maintains better "phase integrity" over stainless rails for a strange reason, the "skin effect", but this is a small difference in DCC, and don't really want to drag this thread down in this unusual and small advantage.
Greg