24P gives a more film-like look to the footage. It's progressively scanned and is recorded (sort-of) at the same frame rate as film (24fps). NTSC Video is interlaced and records at 30fps, which is part of why video looks the way it does.
It all depends on how you want your footage to come out and what you're shooting. A documentary would surely benefit, but maybe you wouldn't want it for news stories or interviews. Go down to your rental house and see if you can shoot some tests and see for yourself.
Please note that 24P footage actually comes into your computer at 29.97 just like regular video (unless you shoot in Advanced mode, see discussion below). The "look" is achieved in camera. And the XL1 and VX1000 do NOT shoot in 24P. You're looking for a Panasonic DVX100A or an XL2.