Guys, this is a relatively easy one.
Movies the world over are shown at a frame rate of 24 fps - that is 24 images are projected onto the cinema screen every second. Movie film is a very high resolution format, far higher in resolution than the DVD format. Accordingly, the movie itself is not the limiting factor in deciding between PAL and NTSC format DVDs, as the same source material is usually used to create both the PAL and the NTSC versions of a particular DVD. Therefore, the limiting factor in resolution is the DVD itself, with PAL inherently higher in resolution than NTSC.
The general rule of thumb when considering the time differences between PAL and NTSC formats is a simple one. Assuming no cuts have been made and the features are identical, the NTSC format runs 4% longer than PAL.
When converting NTSC to PAL, two things need to be accomplished. Firstly, 480 lines of resolution have to be up-converted to 576 lines of resolution, and 30 images per second have to be down converted to 25 images per second.
The resolution up-conversion does not actually add any real picture information to the image because you cannot create real picture information where none existed before. It does, however, make the picture viewable on a PAL display often resulting in a superficially better-looking image, however, if both transfers have been taken from the same source and the conversion process has been correctly applied, the differences between the two are undistinguishable to the naked eye.
The frame rate conversion actually results in a loss of temporal resolution, as PAL has a lower frame rate than NTSC.
Some people have claimed that PAL sources sound speeded up and claim to be able to tell the difference between a PAL and NTSC print. I don't believe that - the human ear is simply not that sensitive.
When converting PAL to NTSC, the converse situation applies with 576 lines of resolution being down-converted to 480 lines of resolution, and frames need to be inserted to go from the 25 frames per second of PAL to the 30 frames per second of NTSC.
Once again, the resultant image is of less actual resolution than the original image, as information is discarded spatially and made up temporally.
Complicating this issue is the fact that more and more programming is being created in high definition and these high definition formats can usually be converted down to PAL or NTSC equally well, with both formats having their respective disadvantages.
It has been established that a PAL DVD has 20% more resolution than an NTSC DVD. This does not necessarily translate into a superior image.
The DVD format relies on a video compression format (MPEG-2) to allow a reasonable length of programming to fit onto a single DVD. The longer the programming, the higher the compression ratio needs to be, and the more likely it is that visible compression artefacts will be present. If an additional 20% of resolution needs to be compressed, then this can potentially result in a lesser quality image if the programming is over-compressed. Fortunately, many PAL DVDs are being produced as dual layer discs, whereas their NTSC counterparts are being produced as single layer discs, thus providing the necessary room for both versions to look their best.
Movies on PAL DVDs play back 4% faster than their NTSC counterparts. The great majority of people will never notice this but strangely for a small minority it's an intolerable artefact.
In conclusion, PAL is a higher resolution format for DVD than NTSC. All else being equal, a PAL DVD of a movie should look significantly better than the equivalent NTSC DVD. If a PAL version of a movie DVD is not 16x9 enhanced and the NTSC version is 16x9 enhanced, then the NTSC version will be the preferred version, all else being equal.
For video-based material it's generally better for the DVD to remain in the same format as the source material. Do your homework before purchasing a DVD! Check as many resources as you can in order to determine which version of a particular DVD is the most appropriate one to purchase.
Hope this helps.