I have a 270 that kind of splits the difference between the two. Very flat shooting, accurate and has a little extra umph for those bigger mule deer. However, I almost exclusively shoot my 243 down here in Texas for whitetail. It is rare to have one go over 150 lbs where I hunt.
Since you are in Colorado, is elk ever going to be on the plate? If it might, then the 308 gets a big nod. Sure the 243 has killed elk, but with elk, I just feel more confident with a bigger stick.
So, if I had to buy just one, it would be the 308, and in the back of my mind, I would plan on buying a 243 sometime in the future.