From what I understand, croc is P2P, i.e. both computers have to be on for the transfer to happen (the "relay" that they mention only helps negotiate the connection between two peers). With aero.zip, you upload your files to a server, and the recipient can download it whenever - either real-time while you're still uploading them (imitating the P2P/croc model), or at a later date. This is a more universal approach IMHO.

Also, aero.zip is a webapp, i.e. there's nothing to install, and you don't even need to sign up to send small files. Meanwhile, croc is a CLI utility which will be hard to use by mom-and-pop users.