Go with a 50 /50 mix but use fat pork. You need the fat to make a decent sausage. If itts too lean it will be dry. Try this recipe

Bratwurst (Oktoberfest)

10lbs. Pork butts
10 lbs. venison
4 cups milk
6 Tbs. salt
4 tsp. Prague powder
1 Tbs. mace
8 tsp. white pepper
1 Tbs. nutmeg
2 tsp. ginger
4 eggs
4 cups powdered milk
Grind meat through a fine plate. Mix spices together and add to meat with the rest of the ingredients. Mix well. Stuff into hog casings. Link and place in simmering water for 20 minutes. Remove and cool under cold running water until cool to touch. Separate the links and freeze. Just thaw and grill.