I live in the United States, so we use NTSC, and my Wii only supports output at 480i (when using a composite or component connection), and 480p (when using a component connection). I played it for about a month at 480i and when I switched to 480p the graphics appeared much crisper on my 32 inch, 1080i HD television (i.e., games with crappy graphics looked worse; I'm looking at you Wii Sports).
I would imagine that using 576i may result in a little bit of a better picture, but according to this thread, it looks like the highest resolution games are made to output at 480, so using 576 you'll probably get some artifacts from the Wii artificially increasing the resolution, kind of like upsampling a DVD to HD (720/1080) resolutions.
It will probably come down to what looks good to you. If you like how things look at 576i, then run at 576i. I would test with some quality games that have good graphics: Donkey Kong Country Returns, Super Mario Galaxy, Twilight Princess, etc. You may not even be able to tell the difference.