In agreement with Tom's comments, the major difference is in the 
thermal properties of the laminate, not only Tg, but max operating 
temperature.  Typical value for FR-4 maximum operating temp is 130C, 
while FR-5 is 170C.  Electrical properties are nearly identical, and 
some physical properties are significantly different.  A solder float 
test value for FR-5 is about 2 times the tolerance of FR-4, and 
dimensional stability of the FR-5 is superior to FR-4.  FR-5 can meet 
many of the tough requirements for military and commercial high 
density SMT applications.

David Mander
Sun Coast Supply