I'm sure someone here will know the answer, and if not, you can try the Classic & Vintage forum. But, for what it's worth, I think the longer arm gives you more chain wrap, meaning you can probably get a smaller front chainring (most useful on a triple crankset) using the second RD pictured than the first. I think the capacity of the low rear gear is dependent on the length of the RD body, or rather the distance between the mounting bolt of the RD and bolt on the top jockey wheel. I'm not sure how you could measure it for capacity without mounting both RD and freewheel to check.