The 20lb is your standard backyard BBQ propane tank. I've never heard of them referred to in gallons, so I'll assume they are the same. I pay $20 for a replacement tank just a few blocks away. If it's half the price for the 100lb, I think it's worth looking into.
I'm smoking two butts as I type, and I just started the smoker with a tank that I don't think is gonna make it through the entire smoke. The alternative of using the full spare tank seemed a better option in the short-term, but long-term, emptying the first tank so I could exchange it for a full one seemed the smarter way to go.
With a 100lb tank, it would reduce the number of times you had to change an empty tank mid-smoke. Save money and reduce the hassle - I'll look into it Monday...
Certainly, a longer hose would be in order.