A monitor is not something to skimp on.
A good monitor will last 10 years without a problem.
Really it comes down to which do you need more? If your disk is causing problems(i.e. way too small) then it's probably more important.
But if you have a crappy monitor, like one of those that cant go above 1024x768, I'd reccomend a monitor.
An HDD will be replaced in a year or two, but a good monitor will last far longer.