i can only speak about the liveview since i have not bought a smartwatch yet.
as you can see within
this thread and on
openliveview.com, custom firmware developement and a liveware manager replacement is in developement.
the custom liveware manager replacement you can find
here, has all the functionality the liveware manager has, except plugin support (wich is currently in developement)
besides, it has a notification implementation that is pretty similar to how custommessagenotifier works.
the firmware is yet not for daily usage since it does not put the watch into deepsleep yet and has no bluetooth support yet.
a big minus would be that you need to kindoff break your wristband holder to charge the liveview.
overall i'd say, the liveview is currently extremely customizable since the custom manager and the custom firmware are opensource.
to the smartwatch:
it has bluetooth 4 instead of 1.1 (liveview) wich makes the bluetooth battery drain pretty small and it could theoretically even be ported to iphone.
the screen is the same except it has multi touch instead of 4 tap keys.
it has more internal storage than the liveview so a custom firmware would be more awesome on it.
overall i'd say, its better than the liveview but unless we devs get our hands onto it, there will mostly not be a custom manager / rom soon, so you will need to stick to connection issues and sony's creepy try to write software.