there is no much diffrence in terms of results but in terms of the time for the compilation there is huge diffrence.XG mode will take very less time compared to the DB mode.
If u want any other information related to the DB mode u can search in the solvnet there synopsys explained u much what are the diffrent advanges u will get from DB mode and what are the chages in the DB mode.
XG is the new mode that handles memory and design differently and is generally faster than DB mode.
Many commands, especially the DFT command also have different syntax, so the migration is not totally straight forwards. Thankfully SNPS gives a tool that translates scripts from one mode to the other.
I suggest you write anything new in the XG mode as after the current release the DB mode will become obsolete and unsupported