Someone correct me if I'm wrong, but PAL 50 has lower refresh rate than NTSC, while PAL 60 should be the same speed-wise, maybe there are color differences, but as far as speed should be the same. Wii is capable of both PAL 60 (480i) and Progressive scan / 480p, which is the better setting available. Both work at 60hz, just like NTSC, and Progressive Scan does away with the interlacing, being clearer. There shouldn't be a reason for someone not to use Progressive Scan (Except maybe a handful of games that for some reason might look better as 480i/PAL 60, or you're hooked on a TV that does not support it, but even then you'd be silly not to use PAL 60 and choose PAL 50).
In any case, in USB Loader GX you can force a certain application or game to run in NTSC on a pal system, is what I do for the american copies I have (precisely of games that are only 50hz in european releases, which are few but some important ones are capped at 50hz, like Little King's Story), or the NTSC version of Super Mario 64 on the Wii VC on my PAL Wii. Sadly I know nothing about Wiiware, the games I purchased are all PAL, just like the console.
Of course, to select Progressive Scan you need to connect the system with at least component cables.
