When alphabets are used in passwords there are 26 options if only lower or upper cases are used or 26 + 26 = 52 options if lower and upper cases alphabets are used. If numbers are used the total number of options is increased by 10.
In a password that is n characters long, the total number of possible combinations with the use of alphabets of both the cases is 52^n while the number of possible combinations if numbers and alphabets of a single case are used is 36^n. As 52^n > 36^n a password created with alphabets of both the cases is more secure than one created with a single case and numbers.
The password created with alphabets of both the cases is stronger.