XOR or add in one time pads?
There is no security difference; there are a handful of practical ones:


With xor, you can have the same code to do encryption and decryption
With xor, you don't have to pick a 'word size'; a larger CPU can handle 4 or 8 bytes at a time, while a microcontroller can handle 1 byte at a time, without changing the ciphertext
With xor, you don't have to worry about endianness (which you would if you, say, added words modulo $2^{32}$


While these are all minor differences, they all point towards xor being slightly better from a practical standpoint