I personally own two Xbox's. One is an arcade, the other is a slim. I use the slim as my main one because of the large 250 gigabyte hard drive; that allows me to install all of my games, making them run of the hard drive instead of the disc tray. That
alone makes it worth it for me; because it's quiet and saves my discs from scratching.
Besides that, the performance is pretty much the same. If you have the extra cash, and want to upgrade I would say, "go for it". You can always use the second console for system link parties, or connect it to a T.V. in another room.
Ethernet cords are not always more efficient. When I got the new xbox I used my Ethernet cord for a few days and was constantly buffering when I switched to the built in wireless adapter the buffering all but disappeared. If you have the money and want to
upgrade I would do so.