something you might want to think about here.... have you considered a 'modular' approach?
Alrobi asked the very good question of whether you were buying one speaker or two. If you are strapped for money to spend and are considering headphones, then I'd also suggest you consider buying one speaker.
It's just a thought but buying one good speaker will give you more insight than two poor ones. When you get enough money to add to your system then you can buy the other... and extend this idea and get a sub at some point too if you think your low end isn't all that hot.
There is a lot to be learnt about mixing in mono too and I would say that would be more valuable than trying to mix on headphones.
Just a thought!