but there are many known attacks which can be used against Tor, plus possible 0-days against the browser, so you can never feel 100% anonymous.
Qubes OS + Whonix is a pretty good alternative compared to just using the Tor browser on Windows.
If you use both of them, you're either important person or have serious concern about privacy and security. Besides, it's not for everyone.
Unlike Tor, you can't access regular website/domain with I2P.