We are generally recommending the MPI Progress Rank runtime for large scale computing (--with-mpi-pr). This gives good performance and is very robust. If you can get it to work, the Infiniband port is usually the highest performing runtime on Infiniband networks, but it has issues if you are allocating large arrays.
The ga++ interface is a lightweight wrapper on top of the core library and the underlying runtime will not affect it much, one way or the other.
Thanks Bruce. The "if" statement reads ominous.