What's the highest gravity beer you plan to brew?
At 1.5 qt/lb, a 5 gallon 1.120 brew takes 35 qts, not accounting for deadspace. Assuming zero deadspace, the most grain you could fit in an 8 gallon is 1.080. Big IPAs can push well past that.
Neither will be able to make a 5 gallon batch of Barleywine.