I had never heard of it until early 1987 when my brother borrowed his mates US NES and brought it home with Super Mario Bros, Ghosts 'n Goblins and Kung-Fu Master to say i was blown away was an understatement!!!. Up to that point i was just playing our family's Amstrad CPC and playing the C64 and ZX Spectrum around mates houses (8 bit micros were super popular in the UK)
I was soo bummed the day my brother had to take the NES back to his mate but got my own that very Christmas, after that i introduced it to my mates and one by one they all go one, im not being big headed but i defo think i was the catalyst for people in my middle school hearing about and getting a NES
But yeah more kids had a Master System, just seemed to be in more retailers and advertised more than what the NES was but that could be down to Mattel having the distribution rights for the first few years in the UK until Nintendo took over the reins, think Mattel only sold the NES to certain retailers but when Nintendo took over it did seem to be better distributed and advertised. I had a Mattel NES