This is the question that every passenger needs to consider before buying a WL/RAC tickets. The possibility of improvement in the positions of the WL and RAC tickets are different in each case.
In case of the RAC tickets
The possibility of improvement in the RAC tickets is much more as compared to the WL tickets. This is due to the fact that more priority is being given tot the RAC ticket holders than the WL ticket holders. In other words, RAC ticket holders can always board a train but they will only be provided with a seat and not any berth.
Generally, a berth in a train is divided into two seats for two RAC ticket holders. In the event when there is a cancellation of any ticket or the quota tickets that remain unsold till the last moment or when there is a free upgrade of any passenger up in the higher class then one RAC ticket holder is given the empty berth. Whereas, the other RAC ticket holder is allowed to take over the two RAC seats as one berth for himself.
In case of the WL tickets
Whereas, in the case of the WL ticket the situations are different. WL tickets, in general, do not let you board the train. Only when your WL ticket moves up higher in the RAC list you can have a chance of boarding a train. This improvement of the WL position generally depends on a couple of factors such as the availability of train, the time of the year, how many days in advance was the WL ticket booked, etc.
One thing to remember is that in both WL/RAC tickets the groups and couples can often get split up in different berths or coaches.