Beef. That way if I want to cook it rare I can. A couple cheap chuck roasts or trimmings off a prime rib will do.
Usually do about 50% beef and 50% venison.
Love the taste of venison in a burger and like lean burger, my 50/50 mix ends up being leaner than any store bought lean burger. Straight venison burger taste like sawdust to me.
I like the taste of pork fat, but our butcher/processor who has a steller rep says that pork fat spoils much, much quicker that beer fat.
We roll with beef fat.
Father in law is a USDA inspector for several lockers around where I grew up and they all use fatty beef trimmings, about 10%. We always ground our own growing up and my family used beef kidney suet fat in the grind; it sucked. Fatty beef trimmings are the way to go IMO if you do want to add something to your grind.
I have done render tests and taste tests on suit to try to answer this from a scientific perspective.
It came down to the basic esoteric answer: depends on your choice of flavor. Scientifically, pork works well with sausages because the flavors blend better with the spices. The pork fat samples I tested lasted 4 months longer in deep freeze than the beef tested...lasted being how it tasted to me after rendered in warm skillet dipped in fresh bread. Beef suit has always agreed with burger, and that might be my own preference.
With blacktail deer, i've used fatty bacon as suit and that rocked my world one year...but disappointed the next. I went back to beef suit.
In my area at least, It's a lot more difficult to source high quality beef fat than it is pork fat. Grass fed cows don't have near as much fat to spare. For example, few of them are fat enough to provide an acceptable "packer brisket" with a fat cap on it. However, heritage breed hogs are abundant and have just as much fat back as commodity hogs.
I'll add fatback to sausages, but seldom employ beef fat. I do occasionally blend some whitetail with grass fed beef for meatloaf etc, but I do that at the time of cooking.
I keep my burger 100% venison (except sausage which gets pork back fat from a local butcher).
Why dilute the healthiest organic meat on the planet? That said I very rarely eat actual venison burgers, I eat a good burger a few times a year and the only way to do it right is ground beef in my opinion. I use my ground vension for many things including tacos, meatballs, pasta sauce, lasagna, meatloaf, fillings for empanadas and wontons. Don't need added fat for any of those.