Don't be fooled -- your machine will bog down with the 5 foot bush hog. I had no problem stalling a L3800 w/ a 5ft before realizing the slip clutch was too tight. You do not have the power to keep a 6 foot deck spinning while tackling moderate brush without working your tractor to death.
Keep in mind that going from a 5 foot to a 6 foot is not an increase of 20% as you expect.
a 5 foot bush hog has a disc area of 19.63 Sq ft
a 6 foot bush hog has a disc area of 28.27 Sq ft
So in short, the 6 foot is 150% the size of 5 foot, and you will regret getting it. Just because the tractor can spin up an implement, does not mean it will keep it spinning when being used.