[slotloco]

Brian, 
Tinned braids are constructed from strands that are coated in a metal similar to that used for soldering, prior to the braiding process. 
One better than the other? 
I have not found one  to work better than the other on steel rail tracks. 
I'm sure others will have opinions. 
Alan

All the good information has already been given, but we need extra humour in these dark days.

Well I can say with near 100% certainty, that tinned copper is definitely better (on almost 50% of cars), and plain copper is clearly better (on the remainder.)
I know this because my most expert customers tell me so.
To identify which is which, take a coin, throw it in the air, and see whether the driver who catches it has plain copper or tinned copper on his cars....
He will give you a concise answer, filled with technical proof, and delivered with deep and meaningful nods and winks. Maybe even a knowing grunt.

I actually sell almost even quantities of each, and my customers are divided into three almost equal camps.
Those who swear by one, those who swear by the other, and those who go "pffffffffffffft,  doesn't make a rats #%@* of difference".

I have been unable to determine whether group A, B or C is faster than any of the others.
Still, I only have about 15 years of data, a difference may yet rear it's ugly head

Oh, me ?  I have both in my war chest, actually three, no, 4.
Both types of Slot.it 0.5 soft, and some really thin Sideways copper at about 0.2mm thick
Oh and some stiff ScaleAuto tinned copper 0.3mm that I only use on really heavy vehicles like RAIDS
I am sitting in camp C having a chilled wine with the rat's posteriors.