Без разницы, думаю. char – это зеркало int. А Unicode отрицательным не бывает. Думаю, это рудимент в C++
На StackOverflow нашёл:If you are using character types as numbers, use:
1. signed char
, which gives you at least the -127 to 127 range. (-128 to 127 is common)
2. unsigned char
, which gives you at least the 0 to 255 range. This might be useful for displaying an octet e.g. as hex value.