Like others have said, spot-shooting is really what you’re doing when deer hunting, so a spot target is fine for backyard practice.
I will say however that a 3D target is much more mentally forgiving, and gives an added confidence boost just before heading out hunting. You’ll find that all those times you’ve been beating yourself up for being a few inches off this way or that equals double-lung most of the time on those 3D targets.
I also like to stick my 3D target back in the woods to practice shooting through natural lanes in the brush. Basically, I’ll plop it down in the woods and walk around the target at different ranges, looking for the best possible shot each 10-20 paces or so. You learn alot about trajectory that way. And you lose alot of arrows! You learn very quickly however what sized shooting “window” at which range is good-better-best. 3D is nice for this because there’s several angles which will never present you with a good shot. Other angles give you a couple of shots. Flat-faced spot targets limit the angles you can take on a target in terms of shooting.
Joel