I have an SX P bass and a Fender MIA P bass.
Both sound quite similar.
The SX sounds deeper and bass-ier and slightly more dirtier.
The Fender is much more even sounding across the register.
Both cop the P bass tone beautifully.
The Fender is more refined sounding to my ears.
Playability is different.
THe neck on the Fender (2008 model) is just amazing.
The SX doesn't sit as nice in my hands.
Overall, both are great basses.
I feel the Fender is definately the better of the two (and at the price, you'd want it to be).
It is more playable, the quality and hardware are nicer.
But the SX is a damn good bass for the money.
I wouldnt sell it for twice what I paid for it, becuase I really think its value is higher than what i paid.
I love them both.
Edit:
I'll also say that when you look at them both next to each other, the Fender jumps out more.
THe finishing of the paint (Candy Cola), the fret board and the neck, along with the nicer controls, tuners, bridge, make it look classier and more higher-end.
It looks like a more expensive bass than the SX