Having the files inside the Library means that they're easier to deal with—one single file contains both the images and the catalog. But, as a single file, it's theoretically more prone to damage. The idea is that a hiccup on the hard drive might make the file unreadable, and the images would be stuck inside the Library forever.
Generally, I recommend photographers manage their images as Referenced because of the theoretical risk to having images in one directory and because it enables a photographer to jump directly into a folder and grab a photo manually without Aperture, should the need arise.
That said, when I'm on location, I usually store my files as Managed, for a few reasons. The first is that I always have one or more backups of my images. Since CompactFlash cards are so (relatively) inexpensive, I now bring enough of them with me to cover a whole shoot without having to reuse a card. That gives me a backup that's nonvolatile and easy to manage, so I don't really worry about the theoretical risk of a corrupted Library. The second reason I don't worry much is that I've never seen an Aperture Library that's so corrupted that it was unusable, though I do know it's possible to have this happen in the case of a major drive failure. But for the shoots I'm doing on the road, this isn't a large risk or an issue (Fig. 4).
Aperture allows users to select their image storage methodology on a per-import basis (images can be switched from Managed to Referenced easily) simply by selecting to store them in the Aperture library, or in another location during import. So when I'm working on a shoot, I make sure I have the Store Files drop-down on the Import Settings/Aperture Library options set to Aperture Referenced (Fig. 5).
Until I change this, any imported images are stored in the Aperture Library and I'm good to go. As I shoot, images are placed inside the container file of the Aperture Library, and I know that, should the need arise, I simply have to grab the Aperture Library and transfer it to another hard drive.
When I'm on a location shoot that takes me far from home, I take a few extra backup steps that aren't necessary if I'm right back at my studio. First, I copy the entire Library onto another drive. This gives me a consolidated backup of the information as of the completion of my shoot. If I'm working with a client, I'll sometimes pass that drive to the client for safekeeping. Usually, though, I'll FedEx the drive to myself, which gives me a backup of my data that's stored separately than the carry-on luggage I bring on the plane.