Q:

systemd service A has `Conflicts=B`.  Both A and B are enabled.  Can you tell which service will be allowed to start at boot, if any, or is it random? 

Installing the firewalld package on Debian 9 also pulls in the ebtables package.   This adds two services to the system, firewalld and ebtables.   As usual, Debian enables both services. 
firewalld.service is marked as Conflicts=ebtables.service. 
What will happen with these two services when the system boots?   Can I tell which of the two will be allowed to run?   Or will neither be allowed to run? 

A:

If a unit A that conflicts with a unit B is scheduled to be started at the same time as B, the transaction will either fail (in case
             both are required part of the transaction) or be modified to be fixed (in case one or both jobs are not a required part of the
             transaction).  In the latter case, the job that is not the required will be removed, or in case both are not required, the unit that
             conflicts will be started and the unit that is conflicted is stopped. 
-- man systemd.unit

In this case, both firewalld.service and ebtables.service are only WantedBy=multi-user.target, not RequiredBy=.
Conclusion: firewalld.service will be started, and ebtables.service will not. 

This does not tell you what happens immediately when you install firewalld.   Fortunately it is the same result, because ebtables is installed first.   This ordering means that ebtables.service is started temporarily, but it is stopped again once firewalld.service is started:

If a unit has a Conflicts= setting on another unit, starting the former will stop the latter and vice versa.