I think you should be using TINYINT(1) because with integer values, you can get values down to 0 and 1. With TINYINT, we only pay with 1 byte of storage. As with CHAR(1), we cannot prevent anyone from INSERTing other values than 0 and 1. But that doesn't really matter, if we're willing to accept that 0 evaluates as False, and all other values as True. Using TINYINT(1), boolean expressions works very well with your column values.

I think you should be using TINYINT(1) because with integer values, you can get values down to 0 and 1. With TINYINT, we only pay with 1 byte of storage. As with CHAR(1), we cannot prevent anyone from INSERTing other values than 0 and 1. But that doesn't really matter, if we're willing to accept that 0 evaluates as False, and all other values as True. Using TINYINT(1), boolean expressions works very well with your column values.

Size, Performance and Indexing:

I feel that retrieving and sorting integer fields is faster than character fields and it's familiar to show 1s and 0s for on/off flags too.