It depends on your goals. If you want an essentially vintage tone, but with higher output, fatter low mids, and rolled off treble, get the SPB-2. I had one for awhile, and it's excellent, but the huge low-mid bump didn't work so well in my modern rock band, and I prefer a more raw, gritty tone compared to the smoothness of the SPB-2. I also like having that "ringing" treble on tap if I want it.
If you want still higher output that's more even across the frequency range, with a hot high end and deeper lows, the Quarter Pounder is a better bet. It's definitely a P-bass tone, but not as vintage to my ears as the SPB-2. I also think it's pretty smooth, but maybe not as much as the SPB-2. (If you want a raw, slightly gritty tone, get Lindy Fralin.) I was planning to get another P-bass to use with a Quarter Pounder, but then I tried a G&L L-1500, and went that route instead for a hard-hitting, full range single pickup bass.