For role comparison both the roles must be in the same system, in same client.
Transaction code SUIM -> Comparison-> Roles
If the roles are in different system, then transport the role into one of the system and do comparison. If no transport connection defined then, you can use the upload and download option in the PFCG.
Steps for Role Comparing:
1. Run the t-code SUIM
2. Go To Comparison and select the option of roles
3. Click on Across systems option it will give option to select the sys name under Remote Comparison there enter the SYS ID between which system you want to do comparison and put the role name in compare role section then execute it will give you the result.
4. If there are any difference between then the t-codes it will be in red color otherwise in yellow?