QRSS or WSPR?

Although WSPR comes in several guises and speeds, I have no idea how QRSS (slow CW) compares with WSPR. Maybe Googling will tell me the answer.

For very low power (QRPP), one of these modes is probably better. WSPR has the advantage of being widely reported via WSPRnet.  Certainly very slow WSPR is probably better than QRSS at reasonable speeds.

The answer is probably the same as, "how long is a piece of string?", but any pointers would be appreciated.