Well, if you want to bi-amp, go for the 3.6, which has a parallel x/o. The 3.7 has a series x/o, bi-amping not possible without surgery.
Another consideration is the possible delamination of the conductive wires on the Mylar diaphragms of all 3.6's. Magnepan changed the glue they used in 2005, so all 3.6's are susceptible to the wires coming loose from the Mylar. 3.7's made before 2005 are as well, so call Magnepan with their serial numbers to check on the glue used in their construction. The delamination can be repaired, but it's a bit of a chore.