300 is a very grainy film, and I have a friend who actually prefers the SD version upscaled because it provides more detail, in his opinion. The Blu-Ray just looks a bit neater, to me, so it's the version I prefer.
But yeah, just about every Blu-Ray I own has a better picture than 300.
For good demo material, I'd recommend the following:
Pirates of the Caribbean 1-3 (particularly 2)
Spiderman 3 (sure, the movie was crap, but it looks stunning)
Any Pixar film
Kingdom of Heaven
Underworld
Harry Potter and the Order of the Phoenix







