FX-37 is one of my favorite developers for TMY in LF. I expose TMY at Iso 500 and get well defined shadow details. I mix it 1+5
I have no experience with FX-39
FX-37 is not a fine grain developer, i have used it for 35mm TMY but that is not a good choice. For LF where grain is not an issue it is hard to beat the increased speed you can get from this developer.
Mixed it also last a long time, i have had a bottle semi full but filled with Argon for about a year and it is still working fine.