I like how it's done in Mobius:
Quoting from
http://www.circularlabs.com/doc/v2/synchronization.htm
6.1 Track Sync Master
To use track sync, you first record a track freely without synchronization. The first track recorded becomes the track sync master and you will see the words "Track Master" appear above the loop meter. Once the track sync master has been set, other tracks that are recorded will be forced to have a size that is exactly related to the master track. If the master track is reset, the next available track to the right that is not empty is chosen as the new master.
6.4 Changing the Track Sync Master Track
Normally once a track becomes the track sync master it will continue to be the master for the entire performance. In some special cases though you may want to make a different track the master. This can be accomplished in two ways: using the Sync Master Track function or the Resetfunction.
The Sync Master Track function will make the currently selected track the new track sync master. This is the most flexible way to select a sync master track.
If you use the Reset function to reset the current track sync master, Mobius will automatically select a new sync master. It will select the first track to the right of the old master track that has a non-empty loop.
Except for that one can also pre set certain tracks a specific synchronisation (Default, None, Track, Out, Host, MIDI).