A lot of people like the P-J combination as a best of both worlds option (including me). On a passive bass, you often see a volume, volume, tone pot set-up like on a passive J Bass.
Some believe there is a degree of tone bleed over, even with either pickup soloed. I have heard that using a 3 way selector switch like the Duff McKagan bass http://www.fender.com/products/search.p ... 0146500323
can eliminate this. I like P-J basses with or without the switch.
Some advise avoiding too hot of a P pickup in a P-J bass, because the P pickup is naturally louder. Opinions vary on this, but it is something to consider. I personally prefer a vintage style P pickup with a slightly hot J pickup in a P-J bass.
The distance of the J pickup from the bridge also has some impact on tone. The same applies to a J bass (with 2 J pickups). I have been happy with P-J and J basses for years, they are all I have ever needed. You can get a great variety of tones with a decent set-up.
The active P-bass special http://www.fender.com/products/search.p ... 0135760306
is another option, but I have no experience with an active P-J bass. Maybe others can provide insights.