There's another consideration I forgot to mention. A really great set of headphones costs less than a really great set of speakers, so fora given amount of $$ the headphones will give a more accurate representation IF (big if) you a) "know" the headphones and b) realize how headphones affect the sound (e.g., ambience is more pronounced, panning is emphasized, etc.).

Once I couldn't afford a set og great headphones but I needed some. Someone sold me a pair of Superlux and I really, really liked them! And they are cheap as chips too. I got a new pair a couple of years ago so now I have one pair for tracking stuff with a mic (closed) and one pair for mixing open. They make wannabe versions of known brands and models. I recomend checking them out if money is tight. I caould afford more expensive ones now but I've stayed with the Superlux - to my ears, they're awesome ;)