I have a an SB3 that I've pulled out of my system, replaced by an airport express. I was having issues with the Squeezebox server software on OS 10.6. It would suddenly "forget" where my itunes library was and I would have to go through the process of rescanning. In comparison, the airport express just always seems to work.
The remote options (if you have an ipod touch or iphone), are also better on the airport express. Not to take away from the amazing work that has been done on ipeng, but it you already have an apple set up, and you are already using itunes as your music catalogue software, squeezebox server is just one more piece you don't necessarily want to fiddle with: the airport/audioengine/ipod/itunes cocktail has been pretty foolproof in my kitchen setup.
That said, if you don't have an ipod, the boom comes with a small plastic remote.
If it's the kitchen we are talking about here, you are trying to set up background music. An airport express into a pair of Audioengine speakers, or a sb boom are both going to be similar fidelity wise. I think the decision should be more based on what you are comfortable tinkering with on the back end. If you already used an airport product (extreme or express) to serve wireless to your house, and you have a way to control the music in your kitchen (ipod) then get an airport express and avoid setting up squeezebox server all together. The linksys is the missing link I'm not familiar with (not to mention the concrete walls). My assumption is that given that all wireless music servers seem to still serve up 801.11g instead of "n", the range of an sb, an itunes express, or an olive 2, will be similar. Will an airport express connect to the linksys wirelessly?
If you already have sb server running on your main system and you want to extend it, or if control via the speakers is a big issue (ie. the ipod/computer are in another pocket/room all the time) go with the boom because the controls are right on the front.