Poseidon is surely more advanced and supports a greater variety of USB classes.
Anaais is just awesome since it can runs ALSO in a plain 68k Amiga (Poseidon needs at least an 020 iirc to run).
Also as you said, Anaais runs on Workbench 1.3 as well.
I'm a huge Poseidon fan, but Anaais is a really awesome alternative that evolves nicely and runs easily in unexpanded Amigas.
You can degrade without the need of switch but then you don't have nice compatibility in certain demos/games that don't like the 030.
In my A600 with ACA630 I have a dual Kickstart (1MB ROM with 1x3.1 ROM and 1x1.3 ROM). With a dual 3way switch I disable ACA and change to Kickstart 1.3 while switching back you get Kickstart 3.1 and ACA enabled.
It's an easy mod that you can check on my blog here: http://mfilos.blogspot.com/p/dual-kickstart-switch.html