1. 26 tires and wheels are more common than 24. 26 wins.

2. Larger wheels are smoother over rough surfaces because of smaller angle of attack. 26 wins.

3. Steel is more flexy than alu, ie more comfortable. 26 wins.

4. Drivetrain parts (no of teeth) are designed for full sized wheels, ie easier to get your prefered ratios. 26 wins.

5. Actually the determining factor is not all the above, it is whether it fits. The your described it is it fits, so the 820 is a better choice i would say. The above points only add to the factors.