2G coverage > 3G
1.You need 3G to 2G HO(IRAT) and IRAT should not be prevented otherwise you will drop the call.
2. I believe 2G to 3G HO should be prevented to avoid unnecessary loading of 3G system . As I mentioned the voice quality is same for both 2G and 3G , both are using same codec amr 12.2 or 5.9.