That is network specific - not all networks do it, and the ones that do it don’t necessarily do it every time!
forum.sierrawireless.com/viewtop … fone#p2990
You really do need to monitor +CGREG; maybe you could also try +CGEV 
What do you mean by “disconnects” here?
Not knowing your application, it’s impossible to say what’s “better” for the particular requirements of your particular application!
Hence you really should be discussing this with your Distributor or FAE - where you will be able to go into the full details of your situation…