It is possible that both backs get better this year. MBIII is going into his 3rd season. He had to have learned a little about the NFL from his last two years that can make him a better back. I don't really care which one is the starter. I do think Julius holds a slight edge as being better based on the last two years in my eyes, but it would not bother me at all if Barber over took him.
Barber was the back up the last two seasons and I think back ups gain certain advantages than starters. I don't know if it can be quantified. It just seems the situation changes for them. With that in mind I am confident in Barber while he is in the game. I never felt that way when watching Troy Hambrick as the back up. He just did not look like a starter in this league. Well, I should say a good starter. He could probably gain 900 yards a year, but that is nothing special. I think Barber can do better than that, but I don't see him as a dominate guy either. He won't be a guy getting 1440+ yards per season, which is an average of 90 yards per game. Emmitt did that for 5 straight seasons.
The reality is both guys are needed and will be used a lot no matter which one is the starter. More of a reality is that both will depend on the O-line improving. The line just has to play better than it did last season.