Very roughly, a factor of '10' between the two.. - Brass being 'better'.
But this is relative between two pretty good conductors. There are other factors, which (can) have a greater bearing on choice of material you use: A coastal location, or area of high industrial pollution, for example.
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.
The black stuff was a source of neverending controversy on MLS & LSC over here, and finally we had it analyzed with a mass spectrometer. Many of use theorized it was carbon from the arcing between wheels and track found on ALL track power pickups... (if you don't believe that, observe track power at the rail at night).
Anyway, the black stuff is oxidized metal, on the brass track, it's composition was exactly the mix of copper and zinc that the rail was made of. It turns out that almost all metals, when oxidized in very fine particles are black.
So no way around it, except to use lower current per pickup point. I find the black is easier to remove on my SS track, which is smoother and slipperier than brass.
It's been fun learning this stuff over the years. There are a lot of "common wisdom" situations where the "wisdom" has been wrong for years. Always good to learn, keeps the mind agile (as the body goes the other way )
If you want to run trains, do it right the first time. I'm with Charlie on this one. I had a small outdoor layout very close to the ocean, and two feeds and screwed fishplates and it was out there for some 15 years. Ran well with no electrical issues. When I pulled it up to re-locate, the track was fully salvageable , fishplate screws and all. Even the rack section was retrieved intact and placed back into the original; plastic bags (with original header cards) Track went back into its cartons.(Serious Freudian personality traits !!) But you are all correct - Rule 8 and love what you do. Now all I need is a yard to set it up again. Not much chance this time around !