Both guns are excellent deer rifles. IMO the .308 is the superior deer rifle. As others have mentioned the .243 is good for varmints, Ill add that the .308 is great for deer, moose, elk and bear as well. The versatility is nothing but a bonus, as I personally think that as far as strictly deer rifles go, you cant go wrong with the .308. I noticed that in a earlier post someone said something along the lines of "as long as your not shooting too far". Im not sure what they mean by that. A .308 will shoot 400-500+ yards with ease if you want it to. At 300 or less, which is more likely for deer, you can put 3 in a quarter with ease.
I would say dont limit yourself with the 243. Its a wonderfull little gun, but lacks diversity. The .308 will out perform it in every way on deer.