In my opinion, Aperture is a little less intimidating than Lightroom from a beginners point of view. I've used both quite extensively and, to me, Aperture seems much more finished. I've not used Lightroom for a good few months, so I'm probably quite out of date now.
That being said, there seems to be a lot more room for extending Lightroom with processing presets, which is quite cool.
Guess it just depends what you want; flexibility/extensibility, go for Lightroom. Simplicity/interface, go for Aperture. I don't think you'll be disappointed either way though.