Do you mean what do I choose to do right now, while I'm sitting in front of my keyboard? Or what will I ACTUALLY do when confronted with the split second decision? Probably two different things. Here are all my vehicle/animal interactions:
Junior in high school, new driver. Was driving down a windy road with cars parallel parked on both sides. A small dog darts from between two of the parked cars JUST before disappearing from view under my hood. Humanly impossible to brake that fast. But I tried. Locked them up. Both wheels hit the poor dog. The lady who was walking the dog came up to me very apologetically "he NEVER runs off like that!" The dog was shaking and twitching. We put him in my car and drove him to my vet. DOA. The twitching was just nerve activity. I took the lady home feeling like CRAP! I wasn't speeding, and she reassured me that she didn't hold me responsible, she should have had him on a leash, etc. But still... the worst thing was, she had her young son (3 or 4) with her. Although I felt horrible, I had to stifle a laugh when, on the way back to their house, she told her son "see what happens if you don't look both ways?!"
The following year, when I was a senior, I was driving thru Waller on Hwy 6 en route to A&M for college visitation. A dog ran across the hwy and again, I slammed on the brakes. The dog got away unscathed, but the car behind me got rear-ended by a trailor-less semi-rig. The car would have then rear-ended me, had I not seen it lurch at me in the rear view and floored the accelerator. (The dog was gone by then.) I probably should have stopped for the police report, but I didn't know how long it would take and I guess I was a little afraid I might get in trouble...
Another time, a well-camoflauged deer jumped in front of me while I was going about 60 down a farm road. I braked (hard, but not hard enough to lock them up) and held the wheel straight. Hit the deer, who fell down and skidded across the road. I came to a stop and got out in time to see teh deer get up and run on. Compressed the gas cylinders on my front bumper (permanently) by about half an inch. Bet it ruined that deer's day, if it even survived the trauma.