Masterblaster1,
From what I gather talking to H&R/NEF Customer Service, both are 150 grain maximum. The longer your barrel, the more effective your burn will be. The 26" barreled version wears the Magnum label in the ads that I have seen.
There comes a point where your charge (usually around 120-130grains) is either propellant or ejecta and if you go over that in your given barrel length you reach a point of diminishing returns. The 26" barrel will more efficiently burn the Magnum powder charges if that is what you so desire.
The 26" barreled version is only about $10 more than the 24" barrel. The 24" will be better in the handling, pointability and balance department while the 26" should be beneficial in accuracy and shooting heavier charges. It is all what your personal preference is.