BT747 now integrates the new map caching method (since 2.X.1616). When launched from the web, the stable version also uses this method.
That should improve performance a bit versus the previous caching method because the file lookup happens faster, but the reason why it got implemented now was a request for cache file compatibility with Marble.
Using symbolic links it should be possible to share the cache files between the programs.
To set the cache location, use the "Set Map Cache" directory from the file menu. That directory should exist (or you should create it).
Using a cache directory speeds up the interaction with the map tremendously - BT747 can use the tiles it fetched in the past rather than waiting for the mapping site to return the tiles. It also allows you to use the map feature offline for places that you visited with BT747 before.
P.S.: Compatibility with Marble is confirmed by 1 user and BT747 is faster than Marble ;-).