In the latest month the gap grew by 227,998 units in favor of the PSP when compared to the 3DS. In the last 12 months the PSP has increased its lead by 1.26 million units. The PSP currently leads by 3.97 million units.
An important note is that the two handhelds launched at different times of the year. The PSP first launched in December 2004, while the 3DS launched in February 2011. The 3DS has sold 74.59 million units, while the PSP sold 78.56 million units.
As time passes it is looking unlikely the Nintendo 3DS will surpass the lifetime sales of the PSP.