Aloha j,
I know both can be used for many different things.
My approach to them is to (try) use
1-‘I’ for jazz/classical/church stuff
and
2-‘G’ for pop/rock/miscellaneous stuff.
Usually works. But sometimes not.
Reason being; that during a ‘heavy mix’, both are sometimes what I call:
‘to big to fit in the hole’ (even with moderate e.q.ing) and in those instances
I will end up using a ‘thinner’ NI piano or HS2 or SampleTank instead.
Good Luck!
{‘-’}