It's a /z/. Here's a simple rule - when you add an 's' to the end of a word, it's always pronounced as a /z/ after voiced consonants and vowel sounds. Those two words both end in vowel sounds, so you pronounce the 's' as a /z/ . "Thighs" rhymes with "size" "...throughs" rhymes with "snooze".

z is the true pronounce