While Java I2P is the original client for the I2P network, i2pd has some big differences and advantages:

• Java I2P has built-in applications for torrents, e-mail and so on, which some may regard has bloat. i2pd is just a router which you can use with other software through the I2CP interface.
• i2pd does not require Java. It's written in C++.
• i2pd consumes less memory and CPU.
• i2pd can be compiled for any device GCC or clang supports (including Raspberry and routers).
• i2pd has some major optimizations for faster cryptography which leads to less consumption of processor time and energy.