it really depends on your tastes. I personally dont like most commercial sausage products as they are just too fatty. I typically go about 50/50~ 40/60 venison to ground pork (butts 25-30% fat, or boxed trim, i think the boxed trim is slightly fattier 30-35%). On products that I want a little less fatty, like summer sausage, snack sticks, etc I will substitute ground beef (usually 85-90% lean) for some of the ground pork, typically about 20% of my meat content.
but honestly, its your sausage, experiment with small batches and find what is right for you and go with it. somebody asked me once why I dont compete in BBQ competitions and my answer was simple, I dont care what others think of my Q. My family and I like it so thats really all that matters to me. ( usually I get my most honest comments from my teenage kids, others wont say what they think, but teenage kids... brutal honesty)
best of luck!