I use Musicnotes.com as my primary source of sheet music, but I discovered this week that buying from them is only good value (for folks outside of the USA) if it's digital sheet music for instant download.
I wanted some music that isn't currently available in digital format, but was horrified when they slapped US$30 p&p on top of the book price.
A quick check with SheetMusicPlus.com located the same music with a much more comfortable shipping cost of US$4.49 (currently NZ$6.32).
I conclude that Musicnotes specializes in digital music, while SheetMusicPlus specializes in paper/books. I'll order my shopping accordingly in the future. = )