Tor and I2P (acronym for Invisible Internet Project) are anonymous proxy networks better known for their popularity among darknet sites such as the Agora Marketplace, although this online black market is one of the Tor hidden sites. Other than the fact that I2P is less popular than the Tor hidden network, it differs considerably from the latter.
Tor uses a centralized directory-based approach in its management of the network. The centralization also enables Tor to gather and report web usage statistics. On the other hand, I2P uses a distributed network approach, which makes it faster and less prone to attacks such as those commonly used by law enforcement to take down Silk Road 2.0 and a number of other darknet sites late last year whereby servers hosting Tor hidden sites were targeted in Operation Onymous.
I2P is designed and optimized for providing quality hidden services. Some aspects of Tor, however, are optimized for other benefits. For instance, Tor uses SOCKS, which was designed to offer great functionality. This is in contrast to I2P which relies on an API, a design feature meant to improve anonymity. This difference in design makes I2P sites better suited to thwart security threats than the Tor hidden network sites.
Generally, I2P is now considered more secure than the Tor network. However, darknet sites such as the Agora Marketplace continue to run on Tor hidden network, and yet continue to evade relentless efforts of the authorities to close them down. Tor leaves a longer and better trail because it uses long lived circuits. In contrast, I2P uses short lived tunnels, and that makes I2P sites harder to take down.
Agora Marketplace is a Tor hidden site that deals in both legal and illegal products, especially illegal drugs and weapons. After the closure of Silk Road 2.0 last year, Agora Marketplace became one of the largest darknet sites, and its continued operation shows the robust privacy and security that the Tor hidden network service continues to provide to its users, even though I2P now generally offers better security.