RLG is heavier and bigger than FOG (about two times).

It is also more expensive than FOG.

Furthermore, FOG is two times more precise than RLG.

Also, the power output of the RLG reduces over time and becomes unusable.

RLG is at the end of its way. There is no reason to choose RLG instead of FOG.

some of the desirable advantages in IFOG are listed as:

a) High rate capability >1000deg/s

b) No lock-in problem

c) Immune to both radiofrequency and electromagnetic (EMI) interferences

d) Negligible g and g^2 sensitivity

e) No moving parts and gas, leading to high operational reliability and long shelf life.

f) Fast activation time

g) Since FOG is photon-based, it is immune to space radiation that effects electrons and the devices based on electronics.

Advantages of Ring Laser Gyro are as follows :

• It gives digital output with angle increments
• It has very high sensitivity
• It has a stable input axis
• easy detection over a wide range of measurement (from 10-3”/h to thousands of “ i s )
• It is insensitive to accelerations as it has no moving parts. Thus the chances of errors due to acceleration are nullified.
• It is very reliable.

There is a good comparison between gyro technologies on this link (there are a few mistakes but not so much. for example it says there is no drift on RLG but there is bias drift due to the gas flow velocity)

https://www.ceva-dsp.com/ourblog/exploring-the-application-of-gyroscopes/

Modern Inertial Sensors and Systems (written by "Amita Bose" ) is a good reference for decision making.