A combo... anything that can be done to make the worlds we are on as "world-like" as possible the better.  If we can have an entire eco-system on a planet I will vote for that.  It increases the ambience of said world and makes the experience that much more realistic. 

I also vote for reactive animals... birds that fly away at the approach of warthogs, tanks and other such noisy units.  Firing of weapons could also shake loose some critters.....

Also... consider space monkies with space bananas....  who doesn't love monkies? Come on....