[krackers]

Protocol-wise they're pretty similar except I2P uses a variant of onion routing called "garlic routing" which allows for message bundling (I don't know if this is currently used beyond bundling for delivery status messages and lease sets). I2P also has unidirectional circuits as opposed to Tor's bidirectional ones which means unlike Tor hidden services where you build a bridged circuit using a rendezvous point, in I2p each side has a pair of unidirectional tunnels for send/receive. I2P is also packet switched as opposed to circuit-switched, which can possibly make it more resilient to traffic-analysis attacks and lead to better load balancing of resources.

Otherwise the main differences seem to be design-wise, where in Tor you rely on a centralized bootstrap to get the list of relays whereas I2P does this via decentralized NetDb.