Different batteries can be a good thing. It can allow you to use the batteries that you have on hand or reduce the size of the back-up. My EDC is either a 2xAA or 2x123 and a 1xAAA. The AAA is only for back-up or second light use with a lithium battery (coin cells are usually nearly dead when I go to use them). By the time the primary light and the lithium in the AAA, most places will probably be out of AA, may be out of 123, but might still have AAA.