What is a colt lower made of?
7175 is about as strong as you can get in aluminum. I believe the ONLY advantage that it has over the 6061 in a lower receiver would be corrosion resistance since both are many times stronger than is warranted.
I'm not sure what 'more rugged' use you have in mind for your receiver, pulling nails maybe? I know in a number of destruction tests, the typical billet aluminum lowers have held up better than the colt lowers and they are what the military has used for years.
Your lower will be stronger than mine, but mine will shoot better and be plenty strong!!
