2 Datacenters with 3 x F5 6900 and 2 x GTM licenses

New to F5 here. We just received our 6900 units with 2 GTM licenses, and was wondering what the pros and cons were for GTM license location on the following scenario:

 

 

We have 2 datacenters, and datacenter1 has 2 x 6900s with LTM failover, and datacenter2 has 1 x 6900 with GTM.

 

 

We are wondering which 6900 to install the GTM license on in datacenter1 as we were told by F5 the GTM can be on either the active or standby unit.

 

 

So what are the pros and cons of having GTM on the active and not passive unit, or install GTM license on the LTM passive unit, so both units in datacenter1 are theoretically active with primary unit serving LTM requests, and the backup unit serving GTM requests.

 

 

Thanks in advance!