Linksys tells me that they are identical except for the 4/8/16 ports that the different models offer. I have read some reports that indicate that there are different versions of firmware for each version and that each version has its own set of problems.
So I read through the manuals and I noticed a few shortcomings. These are features that I would really miss since I use them on my D-Link DI-604: MAC Address Filtering and Static DHCP Clients. The latter is particularly useful since it allows you to assign the same IP address to a device based on the MAC Address. This is very useful when you're dealing with devices that can't be assigned static IP addresses or that have interfaces that are difficult to use. Such devices include Network Music Players of which I have several on my network.
I have also had some terrible experiences with Linksys. Most recently, it was with the Linksys RVS4000 4-Port Gigabit Security Router with VPN
To top it all of, Linksys seems to be getting a terrible reputation. Take a look at the reviews on Amazon and at the posts in various forums.
Conclusion: Linksys routers are *not* the way to go.