Both are good organizations. I would base it on what species you are most passionate about. I used to work for the Mule Deer Foundation 15 years ago and it has come a long way. They are doing some very good things and are on the rise.
Pheasants Forever is a group I have been a member of in the past and they do a lot of good things, too.
Both are focused on sportsmen and habitat development. They also do a good job getting youth involved in the outdoors.
I'm a member of Rmef love all the boats on the ground projects them do. And them help so much with bring elk back to my home state of Michigan.
Pheasants forever local chapters do very good work around here in My area with many kids programs and donations to raffles for the local high school trap shooting teams. I have nothing negative for them