Now, on to the main zipper compartment. This is where the Empire Builder carries its extra space over the Zephyr. Neither bag is skimpy on storage capacity. But the Empire Builder's main pocket is really, really big.
Here is the inside of the Zephyr:
And the inside of the Empire Builder:
The Empire Builder's main compartment is also wider than the Zephyr's. (The plastic file dividers inside the Empire are 16 1/2" wide, about an inch wider than those inside the Zephyr.)
To give a more practical idea of the size of this compartment, the Zephyr can hold this many books, front to back:
The Empire can hold two more:
Either bag can easily accommodate two rows of novel-sized books side by side. So that means the Empire Builder can handle about four more books of this size than the Zephyr can. When thinking about space, incidentally, keep in mind that both bags are taller than a lot of other briefcases out there. Which means that even when the bag is full front to back (with books, for example), there's still plenty of room for things on top of them.
Of course, some of this compartment may be taken up with a laptop and a Brain Cell, so YMMV depending on how much space your computer/Brain Cell combo requires. For example, a 13-inch Macbook takes a size 6 Brain Cell, which is just shy of 2" deep front to back. So with the Brain Cell installed, that leaves about 2" of main compartment for books, files and other stuff in the Zephyr, or 4" in the Empire Builder. (If your laptop needs a Brain Cell larger than a size 4X, it won't fit inside the Zephyr, so the Empire Builder wins by default.)
I didn't include photos of the back of the bags either, because there isn't that much difference between them to see. Except for one thing: I found the big open back pouch on the Empire Builder to be a little too open. I liked the extra space--you could easily slide the Sunday New York Times in there. But the pocket is so big that if there are only a couple of things in there--say a book and a sunglass case--the opening tends to bow out from the bag and rub against my leg while I'm walking. It's a pretty minor thing, but noticeable enough that I wanted some way to fasten the opening to keep things from falling out. That same pouch on the Zephyr is smaller, and lays more tightly against the back wall of the bag--but still holds plenty. Maybe not the Sunday NYT, but certainly Thursday's.
So which bag is for you? Beats me. It all depends on how much you carry and how much bag you want/need to have with you on the go. There is no doubt that the Empire Builder feels larger than the Zephyr, which is noticeably more compact. But the difference isn't so dramatic that one feels like a purse and the other a footlocker. If you are really tall, or if you're used to carrying around a large briefcase, or if you need to haul around A LOT of stuff, the Zephyr might not be enough bag for you. If you are not really tall, or you don't have that much stuff, the Empire Builder might feel like too much bag.
For what it's worth, I'm six feet tall, and for what I usually take with me in my daily rounds--a Macbook, iPod, phone, wireless card, thumb drive, assorted chargers/cables, wallet, sunglasses, keys, digicam, couple of books, spiral notebook, magazines, loose papers, pens, pocket knife, small flashlight, granola bar and an apple--the Empire Builder seemed like overkill. The Empire could hold two of everything I just listed and still have room left over. (In fact, a fairly loaded up Zephyr easily fits INSIDE the Empire Builder, with space left over in the main compartment for four 250-page paperbacks.)
Obviously, the Zephyr isn't that cavernous. But it does hold everything on my list and still has room for, say, two more novel-sized books, three magazines, six CDs in jewel cases, a small digital video camera and a light windbreaker. With all that inside the bag is full to the top (that's where the bag's extra height comes in handy), but it isn't bulging and still zippers easily.
For me, the Zephyr is the perfect size. That said, if I had a 17-inch laptop, or if I had needed a roomier travel bag for airline carry-on, I would have picked the Empire Builder.
In other words, don't sweat it. There are no bad choices here.