I've used both and as far as easy of use, I'd have to say VM. I can't really say on resources because I used Parallels on a PowerBook and it wasn't very fast, but I've been using VM on my Mac Pro and there really isn't a comparison to the PowerBook.
I love VM's ability to unify the current running Windows, so that it acts as if it's part of OS X. I work with a lot of files that are Windows only and I need to be able to use both, with VM I can do that and still feel like I'm on my Mac. I've even styles the windows to match OS X, so when they are running they even look like OS X windows. Great program....
I only use Parallels briefly, so I don't have any positive comments on it. The PowerBook that I did install and configure it on was pretty slow, but the application seemed to run even slower. The setup was crazy complicated, but this was two years ago, so they could have improved that since. I loaded XP Pro onto it when it was up and running and it ran extremely slow, all my boss wanted to do was play Texas Hold'em from one of the online gambling sites, but it would barely run it.
