Numbers that are actually letters
Pyth, 43 - 40 = 3 bytes

?&amp;K@J.HQG-JG"Mix"%"Only %sers"?K"lett""numb


Test suite

This achieves the bonus. Only numbers and Only letters fortunately only differ by 4 letters. printf-style formatting is used with %.

The selection system is done by both taking the intersection of the hex with G, the alphabet, and subtracting out G. If neither ends up falsy, it's a mix, while if the intersection is falsy, it's numbers, and if the subtraction is falsy, it's letters.