Okayy. Time to clear stuff up with logic and reasoning.
I use the P2K. Partially because I have a sweet-ass skin for it, and partially because I like it better. The USP and the P2K are incredibly similar guns, unlike the M4A1 and the M4A4. Although a lot of the choice is up to preference, let's first take a look at the statistics.

Both guns have a movement speed of 240/250 units per second. The both shoot at a speed of 352 RPM, and do the same amount of damage.The both cost the same and provide the same kill award. They take the same amount of time to reload and. finally, have the same amount of armour penetration.
The USP-S has an accurate range of 29m, and has 3 magazines of 12 bullets each. For programmed values, it has a spread of 1.5, firing inaccuracy of 71.00, moving inaccuracy of 13.87, recoil magnitude of 23 and no bullet tracers.
The P2000 has an accurate range of 31m, and 5 magazines of 13 bullets each. For programmed values, it has a spread of 2, firing inaccuracy of 50.00, moving inaccuracy of 13, recoil magnitude of 26 and has bullet tracers.

Now, when we compare these two guns in their differing values, you get the following:
Bullet Spread - USP > P2000 (has a tighter spray)
Firing Inaccuracy - P2000 > USP (bullets are closer to where your crosshair is)
Moving Inaccuracy - P2000 > USP (bullets are closer to where your crosshair is when moving)
Recoil Magnitude - USP > P2000 (recoil is less when full-spraying)
Tracers - USP > P2000 (USP has no tracers through smoke or wallbangs)
Ammo - P2000 > USP (P2000 has more ammo and larger mag size)
Accurate Range - P2000 > USP (P2000 is more accurate at a longer distance)
When this is all tallied up as a point for each category win, you get the following:
USP-S - 3
P2000 - 4
Now, before you all conclude that the P2K is better, I just want to add that I think the damage drop-off for range is less for the USP, meaning that, at longer ranges, it will do more damage than the USP. However, I could not find these values, so I left that category out. With it in, it would make the tallies 4-4.

So what's our conclusion here? There is a difference, albeit slight, but that difference is mostly based on preference. The P2000 has more ammo, is more accurate while moving and standing still and has a better accurate range. The USP-S has a tighter spray-pattern, less recoil, no tracers, and possibly does more damage at longer ranges.
The choice is yours, as you now have the stats. It's mainly preference, but you can see the the main difference is the P2000 is the more-accurate weapon, with more ammo to make up for it's lesser spraying, while the USP-S is better at spraying, and has less ammo to make up for it's better spraying.
Do what you want with this information, but these are the facts, and shows the very minor differences between the guns. In the end, you're better off at judging your favourite weapon via your playstyle and your skins. Do you prefer to tap, or are you better off at spraying? Would you rather have more ammo to spray with, or are you more in it for the spraying accuracy? These are the questions you should ask yourself to chose your favourites.