• This has nothing to with Tor being attacked at the moment. The decision far predates these events.
• i2pd serves the same efforts as Tor, but on a more p2p level, rather than relying on servers. We are serious about decentralisation.
• i2p lends itself more towards our workload.
• Monero i2p nodes will also act as general i2p routers, which increases the size of the i2p mixnet and thus has an upshot for both.
• Tor is optimised for low-bandwidth clients and high-bandwidth exit nodes, whereas i2p is optimised for internal hidden services. Thus, i2p is significantly faster when routing internal traffic.
• i2p's floodfill routers (roughly analogous to Tor's directory servers) aren't hardcoded
• i2p is a packet-switched network (as opposed to circuit-switched) which makes it more robust
• no client-only peers, all peers route traffic and assist in building and running short-lived tunnels
• TCP and UDP are supported, which means that things like OpenAlias can still work over i2p

Check by yourself: https://github.com/PurpleI2P/i2pd

Sources:

• https://bitcointalk.org/index.php?topic=583449.msg10022918#msg10022918
• http://www.reddit.com/r/Monero/comments/2ti53m/why_is_monero_aiming_to_integrate_i2p/cnzamud

Update: https://bitcointalk.org/index.php?topic=583449.msg11055984#msg11055984