If you are using progressive scan, then it doesn't make a (noticeable) difference.
If you are not using progressive scan, then PAL (R4 as well as R2) delivers a better image (in terms of resolution and colour), all other things being equal. Sadly, all other things are not usually equal, and the image is affected by different transfer processes - so although a PAL image will theoretically be better, in practice it might not be because the DVD may have a poor transfer making the R1 NTSC image better. However, I can always notice the scan lines on an NTSC image, but I cannot see them on a PAL image. The scan lines are more noticeable on non-anamorphic NTSC material but are still visible on anamorphic NTSC too.
Also to consider: PAL suffers from 4% speed up over NTSC (not visible/audible to me but I've never done a direct compare), but NTSC suffers from 3:2 pulldown (i.e. "jerky" panning, slighly noticeable by me, but only when I look for it).
It's complicated stuff and sadly can't be reduced to a yes/no answer. To make matters worse, when you choose to buy a DVD you also need to consider other factors such as censorship differences between regions, differences in extras, and so on. Good place to start when comparing regions is
www.dvdcompare.org.uk
