Hi ,
when you ask SAP, the answer is "Single Stack". That is the actual strategy.
But we have this strategy already seen to change very often.
I personally prefer dual stack, because it keeps you all the options open. Especially if you have a lot of IDOC/ALE scenarios, the ABAP-Stack is helpful. It is a more conservative approach to things and BPM is a huge migration topic. The rule is "Don't change it when it works"
On the other hand, the Java Stack (at least from what I have experienced so far) seems to be a good performer. Some very large customer have already migrated to Java only and I was suprised how well it worked, even on a large scale.
It keeps Operating and Administration much leaner than before.
So, my advice: If you are conservative, have a stable environment: Stick with the dual stack.
If you can afford the change (project, timeline, ressources) go with the Java stack.
HTH
Holger