1.) Does either of them implement an anti-virus program (or can you buy one to install)? If so, how good is it against fighting off viruses, malware, etc?
Clam and Antivir are the only ones I've ever used. They should be available in Ubuntu. AVG has a linux version also.
2.) Does either edition have a firewall app included with the OS? If so, how well does it work?
I don't know of a distro that doesn't come with a firewall, so yes there is a firewall.
3.) Can you network (using peer-to-peer networking) a Ubuntu Desktop and/or Server machine with Windows machines? How about with Macs?
Yes. Macs will probably connect via a CIFS connection and there's plenty of articles on how to connect via a windows machine or mac and how to share your stuff.
[quote="cbassett01"]4.) Does either edition of Ubuntu have any type of media streaming apps (or something similar that can let you stream video to a TV or portable device, etc--or even something similar to Media Center in Windows)?
There is, but I've never been impressed with any of them and don't use something like that enough. Maybe someone who does more with that will add to this. I suppose you can use VLC or MPEG4IP.
Personally, I'd probably go with the desktop version, unless there's truely something compelling you to get the server version....and no, the fact that it's branded as server is not a reason