MacUpdate's download link for iTunes 5.0 comes directly from Apple... this is the exact link MacUpdate uses (althought the status bar download link appears different, because it's a redirect to Apple's own page):
http://appldnld.m7z.net/content.info.apple.com/iTunes5/macosx/061-1813.20050907.tzsmc5/iTunes5.dmg
This is the link directly from Apple's site:
http://appldnld.m7z.net/content.info.apple.com/iTunes5/macosx/061-1813.20050907.tzsmc5/iTunes5.dmg
Any perceived speed increase of using MacUpdate over Apple's own site is just that: purely perceived! The downloads are coming from the same place...
You can do this test yourself: download iTunes from MacUpdate, and right-click the file in the Downloads window and select "Copy Address" and paste and compare.
While MacUpdate does sometimes offer mirror download links to certain files, Apple keeps heavy track of the number of iTunes downloads and would only allow iTunes to be downloaded directly from them for recordkeeping.