The 300 Spartans is almost certainly a better movie than 300.
Mind you, somewhere between the two approaches to the story lies the truly great movie that both of them fall short of being.
The 300 Spartans stars Richard Egan as a handsome, if not as sexy as Gerard Butler, Leonidas, along with Sir Ralph Richardson, Diane Baker and David Farrar. It is a much more complicated telling of the story, allowing the viewer to understand what the battle respresented, why it was important in the general scheme and how important strategy was to the Spartans partial victories along the battle.
300 too often falls into the faulty, and very reflective of modern America, philosophy that being "psyched up" and "right" is somehow the key to victory, or partial victory. The 300 Spartans, in this, is the more adult approach, although certainly not lacking for chest-beating "We are Spartans!" moments itself. Knowing instead that both passion and intelligence are important on the battlefield.
There's also something that having 1000 extras standing on a battlefield from opposite directions that CGI rarely manages to substitute for. The exceptions, I believe, still manage to have hundreds of extras and the CGI stand in for the thousands behind that. It not only has a gravity and weight, but a clear purpose.
300 fails miserably in making clear the brilliance of holding the Persians off at the pass at Thermopylae or capturing the danger of the Persians coming around the other way. All of the battles have a certain level of sameness, slow-motion and medium shots, coupled with distant shots only from certain points of view.
The 300 Spartans also manages to show a far more compelling Xerxes. A power hungry man, intent on fulfilling his own quest for power, rather than a peculiar fey demigod.
Side note: The DVD comes with a Spanish trailer. I don't know why, as it is, as far as I can tell, exactly the same as the US trailer, except... get this... it's in Spanish. I'm pleased they do, however, because the Spanish announcer says Xerxes in Spanish, which is something roughly like "CARE-kess", and I've now taken to walking around saying that randomly to my own great pleasure.
Anyway, 300 shows rather than tells more of the details of horror and sex, that The 300 Spartans is forced to hold back on. It also is superior in terms of the integrity of the passionate warrior feel of the Spartan warriors. It also certainly manages a more striking and beautiful look, and moves along with a great deal more energy.
Like I said, somewhere in the middle is a movie, unmade to my knowledge, that takes elements from each of these solid but flawed movie and comes out with something truly, astoundingly great.