When commenting a selected block of code, Qt Creator behaves differently depending on the shape of the selection:

• // if the cursor is in 1 row and the selection is empty
• // if the selection spans any number of rows but only full rows (ish) are selected
• // if selecting only indentation space
• /* */ otherwise

This is undoubtely smart, but it can also be quite unpractical because you need to be careful when selecting the code.
IMHO it would be nice to choose if use only // comments or both.
It could be a simple checkbox in the Text Editor -> Behavior section, as in the mock