1) afn is somewhat of an anomaly. When I see "fn" I know "this is a function" but when I see "afn" I have to pause for a split second to realize what it does.
2) Pattern recognition due to redundancy. When I glance at the first version, my eyes quickly notice that there are 3 references to "rec", and thus I immediately form a pattern in my mind. But in the second version, there are two references to the same function: self and rec. This adds an additional cognitive overhead for me.
In other words, the first one appears less cluttered to me because it has fewer unique elements, and because "fn" is more familiar to me than "afn".
It's not a huge deal, but I would like to reduce the cognitive overhead of reading and writing code as much as I can, which is one of the things I really like about Arc: it's clean, simple, and short, but allow tons of flexibility to change the language to suit the way you think.
