Well there are multiple considerations. One is, "what makes the best film?" Nolan knows how to make great films, and understands this very well. Another is, "how do I best do justice to the source material?" Nolan seems to disregard that, and Snyder seemed (with Watchmen) to maybe go a bit overboard in that department, though I think it is the right approach when tackling Alan Moore, because he's the man and folks shouldn't **** around with his work too much (and much better than having him turn it into Sucker Punch lite).
There are different approaches. The best in my opinion is where a filmmaker takes much of the essence of the comics (which is what many of us know and care about to begin with) but gives us a new story on the screen in a way that works as a film, tweaking aspects of the comics that don't work well toward that end. I personally think X2 and Spider-Man 2 are great examples of this, even though both have their flaws and detractors.