Basic differences between Tor and i2p: https://github.com/prayank23/Learning-Bitcoin-from-the-Command-Line/blob/i2p/15_0_Using_i2p.md
Only Tor or i2p
Tor/i2p and clearnet peers
onlynet=onion configuration option can potentially ensure the node attempts to only
connect over Tor. It is more private when you combine it with no reachable IPv4/IPv6 address,
in particular if you want to broadcast transactions without them being correlatable with your IP.
On the other hand, if you only make random Tor connections, you're much more vulnerable to Sybil attacks.
As Tor addresses may be created at no cost, an attacker can potentially flood the network with many Tor
nodes and receive all of the outbound Tor connections an
onlynet=onion node makes.
This is significantly less a concern with if you make
-addnode connections to trusted peers
(even if they're onion addresses). It's also alleviated with IPv4/IPv6 (especially when using the -asmap
configuration option) due to the cost of obtaining IPs in many networks.
If all of your connections are controlled by a Sybil attacker, they can easily prevent you from seeing confirmed
transactions and, with more difficulty, even trick your node into falsely reporting a transaction as confirmed
on the blockchain with most cumulative “chainwork”.
If too many nodes use
onlynet=onion, it could become difficult for onion nodes to communicate with clearnet nodes,
preventing the Tor network from seeing recent transactions and blocks. It is essential that some nodes access both
clearnet and Tor.
Tor + i2p + ipv4/ipv6
If looking for privacy, this may not be the best option however I don't see any security issues with this setup.
Tor + i2p
I would consider this as experimental for now and will update the answer if I find any issues.
For maximum privacy, it is preferable to disable accepting incoming connections.
Related Question: https://tor.stackexchange.com/questions/6930/tor-i2p-on-a-system-and-is-i2p-better-than-tor