Hmm, you are talking about Surface RT, right, not Pro?
If so, the Surface does everything your iPad does plus more, built in. -Having the ability for "home groups",
-creating networks by command lines,
-having Office for when you need it.
-Its UI is definitely refreshing.
-Full size USB port
-Full integration with MS services (the new Bing search is amazing)
-built in remote desktop
-definitely, the ability for "true" multitasking. Trust me, its a life saver
-Windows explorer for managing your files or files on other pcs in your home.
-you can map storage,
There's alot more, but I'm getting tired lol.
The benefits for your WP would be easy file management, using explorer or the desktop app provided in the marketplace. (windows explorer is better imo)
-having your calendar,contacts, tasks, email, etc in constant sync (if you choose).
-obviously, similar interfaces
Hope this helps, ask more if need to.