I can tell you that the PIX is a nice piece of equipment, and is always my first choice. However, they're not really geared towards the uninitiated. If you have a complex networking environment you'll appreciate the products flexibility.
Unfortunately, I've never used Watchguard. So, I can't really help you there.
As an alternative, have you looked at the SonicWall line of products? I've implemented a few of them. You don't have to be a network engineer to set them up, and they come with a fairly good manual for someone not familiar with firewall configuration. The only downside is that they are less configurable than the more complex products and you may outgrow it's functionality as your network gets large.