I guess the startegy is to keep the two network as independent as possible , unless it is necessay to save a call due to lack of coverage(IRAT)/Capacity(ISDR) .
Both 2G and 3G are using 5.9 amr so voice quality is the same.
Howeve the edge data user have to go have to idle mode reselection to 3G.
So it is important to put the 3G access parameter to floor , so it wont select 2G untill 3G is dead