In this article, I’m going to talk about engine cases, and which is the best for your build. Basically, you had the 36hp, 40hp, 1300, 1500, 1600 single relief and the 1600 dual relief. The first thing you need to do is decide what are you wanting to build. Is this a restoration? Is this for a reliable daily driver? Is this a weekend hot rod? I will do my best here to explain the differences that should help you choose what to get.
All the stock cases are made out of a mix of aluminum and magnesium. This is typically a good thing. Magnesium is light and a harder material than aluminum. Aluminum is softer than magnesium and heavier. Aluminum is also cheaper. Mag gives the block some hardness which is good when you have bores involved because it will have a tendency to hold its shape. The problem you might run into is that if there is too much mag mixed in, then the case can be brittle. Think about it this way: A cam is super hard and lasts a long time because so, yet if you drop a cam on the ground it can break in half. Make sense? So, too much mag and the case can crack, but too much aluminum and the bores will be softer and not hold their shape as well.
If you are building a bone stock motor in the 36hp or 40hp group, you don’t have much of a choice but to use the stock case. This is fine as both those motors had low compression ratios and don’t produce the heat that a higher compression motor does. As the cc increased in the later years, so did the compression ratio. In 1971 VW, had to rethink their blocks and that’s when we got the dual relief blocks. It was a vast improvement over the single relief blocks.
So what did they do to make it better? First, they cut down on the amount of mag they used. Why was that good? Well, we have all heard of the cases cracking down the back behind number three cylinder. We can all agree that aluminum and mag expand at a greater rate than steel. We can also agree that it will contract (shrink) at a greater rate than steel. So here is what happens: The motor gets hot, both the case and the steel cylinders are expanding. No problems yet. The problem is when the motor cools down the aluminum is shrinking faster than the steel cylinder sleeve. So, the case is now shrinking on an expanded steel sleeve. The case is thin behind number three cylinder and in a single relief case it is more brittle. Something has got to give and it is always the case, because the steel sleeve is stronger than the case in that area. In a dual relief case, the mix of aluminum and mag makes it more pliable and not so brittle. Can you crack a dual relief case behind number three? Sure you can, I have seen it many times. Every time I have seen it, it’s because the engine was way overheated and again, something has to give. 
The second thing that is better on a dual relief case is the oil passages. If you were to compare, you would notice that in a dual relief case they’re twice as big. That is good, really good! Oil is the life blood of a motor. Having bigger passages means more oil can get to where it needs to be.
Third improvement, the webbing in the case around the main bearing area is a lot bigger. This is good because the center main needs all the help it can get. That is why shuffle pinning the center main is so good, it gives it even more added support. Keep in mind here that the center main in all of these cases is kind of an island, with little support at all on either side. It also does twice the work of the other main bearings. While the other two big mains feed the crank and one rod, the center main feeds the center main bearing and the two inner rods. That webbing is a big help.
So what about the fact that it has two oil reliefs? Well, that’s nice but is not the biggest factor in why dual relief is better. It is for the reasons above. Here is what the reliefs do. The one by the front pulley determines where the oil is going to go. If the oil is cold (thick) then the piston goes down and a majority of oil is directed into the passage that feeds everything. If the oil is warm (thinner) then the piston stays up and the majority of the oil is now directed to the passage that has the oil cooler in it. Once it runs through the cooler, it hits the same passage that the oil was in when the oil was cold. The rear piston just release any excess pressure back in the sump.
Last question here I am going to get is “What about the aftermarket all aluminum cases?” I myself prefer those for a weekend hot rod or drag car. They are heaver and have a lot more material in them. A weekend car or drag car will never see the miles that a daily driver will. 
So there you have it. If you are building a bone stock restoration motor then I would use the stock case. If you a building a daily driver and want it to be as reliable as you can then I would use a dual relief case.