My opinion is - it all depends on what you want to do with it.

If all you are going to do is make hamburger patties, then you use beef fat.

If you are going to make sausage - then you use pork fat.

Bacon is a nice additive - down south carolina - they put bacon in deer meat along with some beef fat and it makes it taste just like a bacon burger you get in a restaurant.

The way to make good sausage is to have a recipe and ingredients that you might be able to buy off your local butcher that is the same stuff that your local grocery store uses to make their own sausage.

This is the stuff that I use
http://www.conyeagerspice.com/hunter.htm

i ve always used beef fat so the burger can be cooked to medium and not have to worry about undercooked pork

I just buy the cheapest hamburger I can find at the local grocery store. Last batch was 73% lean (i.e. 27% fat).

I mixed 5lbs of burger per 10lbs of ground up trimmed elk to get a final ratio of about 90% lean ground meat.

This is a good thought, but you can eat undercooked pork.

I love the bacon idea though!

make bacon burger, i get about 5 deer done of it every year. its great

Can't beat the bacon burger, seriously, try it.

The local processor here usually adds about 15% beef fat to the mix. When I make deer burgers I add a little bit of barbecue sauce (1/3 cup for 2 lbs) to the meat and it tends to sweeten up the meat. Mighty tasty!