There are a few threads (started by me

) on the W960i.
They are almost identical phones. Almost being the operative word. The P1i has infrared, where the W960i doesn't, but the W960i has 8GB storage (but no memory card slot) while the P1i comes with a paltry 512MB card. They both have the familiar jog wheels, but the P1i has it's "Back" button on the side while the W960i has the "Back" button on the front. A nice touch on the W960i is that the screen is more flush with the rest of the face. This might attract more scratches (shouldn't be too much of a problem unless you're throwing it about), but it makes using the interface with your fingers easier (the P1i makes it a bit tough to get to the on-screen buttons in the corners.
Functionally, though, the phones are very very similar. If you are looking for a good camera, they both have 3.2MP camera's, but the P1i has autofocus, while the W960i doesn't.
The stylus that comes with the P1i is also a bit nicer than the W960i's. This is because it incorporates more metal into the design (the W960i's stylus is, I think, completely plastic).
That said, I'd still rather go for the W960i. It has way more storage, looks better, has a more accessibly screen (due to it not being indented in the face of the phone) and generally feels a bit nicer in my hand. And I can live without infrared (never use it these days: rather bluetooth), autofocus (it's a music phone, not a camera phone... Even the P1i is a business phone, so the autofocus, while nice, doesn't guarantee amazing photo's), a side-positioned "Back" button, and a QWERTY keypad (I actually prefer the regular keypad to a qwerty pad).
Actually, that last point brings me to one of the biggest pitfalls of the P1i: People will tell you that you quickly get used to the iffy keypad (in case you don't know, it's a qwerty keypad with a twist: 2 keys share one physical button... sort of a rocker-type system: left side of the button does one thing, while right side does another), and that may be true, but they're also the ones that are SLOW with their typing. So while it may not irritate you as much after a while, I don't think proficiency increases much over time.
If you are definitely looking for a Sony Ericsson smartphone, the W960i is definitely the way to go, but at the price that is being asked, it's currently just not worth it (try MTN, they might cut you a deal), especially with the iPhone 3G around the corner! The P1i is (in my opinion), a more sensible W960i, but even at the (slightly) lower price, it doesn't seem worth it... Bear in mind that last year, the P1i was on offer for R299-00 pay-in on a Vodacom Weekender Everyday, but now it's much more!
Hope this brief explanation helps you!
Oh, and to answer your original question, they're both excellent phones. It's just the price that puts people off!