Thanks Cato1507. I came to the same conclusion after visiting my local brew supply shop. He sold me on the idea of a 16 gallon pot for 5 gallon.
Also, the batch I recently made resulted in the decision to go with the larger 65L tank and here's why:
I started with a 13 pound grain bill that required 9.25 gallons of water to include the sparging water. On this recipe, I used a 16 gallon pot I had as well as a 9 gallon pot and a 5 gallon pot. I put 5.75 gallons of water in the mash tun (9 gallon). I put 3.50 gallons of water in my sparge pot (5 gallon). After mashing, I transferred to the larger 16 gallon pot and added the sparge water before starting the boil. When I was done and turned off the heat about 90 minutes later, I was just under 6 gallons. After transferring this to my fermenter I realized that I had a LOT of hops and solids at the bottom of my fermenter (which made me wish I had a filter somewhere during the transfer to the fermenter rather than just dumping it in). After a week I transferred to the secondary and I was just under 5 gallons. I added another 2oz of hops directly to the secondary and let it sit for another week. Once that was done, I transferred to the keg and let it sit for a day at 30psi before chilling it. I ended up with about 4.5 gallons of finished beer so my next batch will have to be closer to 9.75 or 10 gallons of water. I realize that I could just add water to the fermenter but I like the fact that I can do the entire brew and transfer over from one vessel which is suppose to be the point of these all in ones right? Also, if I understand correct, I don't need a sparge tank as I can skip the step of sparging...correct?
My efficiency using the 3 pots and propane was 73%. That's the first time I've ever brewed an all grain recipe so I call that a success. I have materials for another batch but I'm waiting on the Brewzilla to arrive as the 3 pot solution took about 3 hours of prep time just to clean.