It would be clearer if you mentioned the fact that syncing with non-Ubuntu systems might be desired by the user. If Ubuntu One was the right choice for syncing, I would think it would be better to just install it, even though it may not be used, than ask users a question they may not understand.
A (possibly) better procedure would be to describe briefly the function of syncing during the install, and give (at least) three choices:
Ubuntu One (the default) would allow syncing between Ubuntu (or all Linuxes perhaps?)
Dropbox would allow Windows machines also to be synced, but have non-free complications
Inclusion of Dropbox as an option might be a hard sell or even impossible, but I'll bet if it was included someone would find a way to provide a Linux-oriented service soon, perhaps interfacing with the Samba package.
Over the internet, or over a LAN?
On the internet, there are a number of file servers you could use that could be used with any OS.
On a LAN, just get file sharing working, and set up a shared folder that all the computers have access to.
Why make it complicated?
Yeah, Ubuntu one probably won't include Windows support so DropBox (or other online file service) would come into play. I didn't want to share files via LAN, although it would work. Here's what I usually do:
New school project, and I start working on it on my laptop. Save it into the Dropbox folder and it syncs with my PC at home so I don't have to transfer the file when I get home. The same goes for my PC to my laptop. I also love the feature of being able to access the file from the web. The thing is, both my PC or laptop aren't usually on at the same time so I love how it stores it onto a remote server.
I don't believe that there should be a option for DropBox or other service, as I don't believe many people do these types of things. (that's my opinion on it)
I'm not asking for help, I'm asking for a vote (of your opinion) on which you think is good for Ubuntu.