Let's suppose that with Zack Snyder's direction, '300' conforms to Miller's version as depicted in the 5-part comic book series. Much has changed over the years in the way war has been presented on film and in my opinion there is no better way to illustrate these differences than to compare 'The Longest Day' with 'Saving Private Ryan'.
The 'Longest Day' which coincidentally was released in 1962 as was 'The 300 Spartans' was filmed in black and white and was based on the true story of the D-Day invasion during WWII. Now we fast-forward to several decades later and we have 'Saving Private Ryan' which is a fictional account of the D-Day invasion, loosely based on the Sullivan brothers who perished during WWII while serving in the Navy. Much of the movie was filmed with muted tones, perhaps to minimize the impact of seeing all the blood and limbs strewn over the Omaha beach head, however, it was still an extremely graphic movie, especially the first several minutes.
The 'Longest Day' and 'The 300 Spartans' were sterilized accounts of the battles of D-Day and Thermopylae, respectively. In keeping with the times, the wounded and dead were shown in sanitized versions, without any limb dismemberments, nor with any blood spewing from wounds. In other words, these visuals were conforming to the standards that had been set by the motion picture industry for the early 1960's.
'Saving Private Ryan' which premiered in 1998, on the other hand, showed several of the GI's with ghastly wounds and other horrific scenes with which I had never seen in other WWII movies. It also showed the aftermath of the battle which for several seconds focused on the bloody tide gently cascading over the American dead. In other words, the battle scenes were very realistic and extremely explicit even in these times of relaxed movie standards.
Therefore, I believe that the movie '300' will parallel and perhaps surpass 'Saving Private Ryan' in its depiction of the brutality of war. 'Braveheart' and the 'Gladiator' are other examples that show how gruesome war can be, so I suspect that based on Zack Snyder's track record, '300' will be as bloody, if not bloodier.
Would you rather see an accurate depiction of the battle, or would you rather see a movie with very little blood and gore?