Honestly, if you post a "What bass to buy for $1000," you will get 1000 different answers and then some more. Best advice I can give is to try some basses out in a good music store and see what you like.
The used market opens up a lot of options but you may need to buy the bass w/o playing it. I'd check the used Stingray, Sterling, and G&Ls out. But I really like Fender styled basses for slap stuff also.
The differences between the P and Stingray/Sterling? The MM basses are active so you've got more knobs to dial in your tone. Unless you went for something like a P Deluxe, the P bass only has 2 knobs for volume and treble so all the tone is in your playing style. Also, the MM pickup is closer to the bridge which allows for a "snappy" sound and a nice, tight attack IMO. The P bass pickup is in the middle of the bass and gives out a real woody, warm tone though you can make them pretty bright if you try!
Hope that helps!