Personally, i think that Caesar's Legion would win in a full battle against the NCR. Let's face it, the NCR may have slightly superior numbers, but they have sloppy and poorly trained soldiers (with the exception of the Veterans and 1st recon). Have you honestly, even seen any NCR soldiers training?
On the other hand, the Legion have physically fit and skilled fighters. If you enter the fort, all you see near the tents is Legionnaires training. They are rather fast and accurate with their attacks, compared to NCR troopers who happen to miss half the time.
I'm just saying that, if every Legion soldier in the East and West went up against every NCR soldier, they would probably be outnumbered, yes, but in the end i think they would be superior. What do you think?