I have both. I recommend the P3P for people buying their first Phantom, here are the reasons:
1. Cheaper. You can find them used for a good price, with lots of worthy accessories that add up.
2. Video quality is virtually identical. Yes, the P4 camera has better glass, but I can't tell any difference, they both look GREAT.
3. The P4
has a weak RC, the P3P has better control transmission and range in stock form. Use a $15 windsurfer with P3P and you're good for over 2mi.
4. If you're a beginner and crash, you can get the P3P fixed cheaper than P4, due to the complexity of the P4.
5. I rarely use OA on my P4. Only when I get in close to an object via FPV do I enable OA. With OA enabled the P4 will only go about 17mph, which is too slow.
6. By the time you get good with the P3P, you'll be ready for a P5 next year (likley Mar-April timeframe) which will hopefully have better range than P4, among other new features. But wait till summer to buy it so the bugs get worked out (SOP for DJI ;-)
FYI, I didn't fly my P4 for months because of crappy range, I'd always fly my go old reliable P3P. It wasn't until I linked an Inspire controller to my P4 that I started getting better range, comparable to my P3P. I use the P4 most of the time now only because it flies 3min longer than the P3P, which comes in handy on longer flights.
There is nothing wrong with a P3P, they work great. I haven't sold my P3P because I like it so much, I'm emotionally attached to it because we've been so many places. I use it as backup.