If you are planning on a no-sparge BIAB (Aussie style), then I'd go with the 40 qt. I have a 32-qt and I frequently push it to the limit with just 4 gallon batches. Bigger beers or the occasional 5 gallon batch forces me to do a dunk sparge in another vessel. If I were to do it all over, I'd have gone with a 10 gallon/40 qt kettle. If you are planning on sparging and have another kettle to heat sparge water/dunk grains, then either should be fine, otherwise get the bigger one.