I'm trying to figure out what's best, as I only have 5% karma.

It is based on number of offerings, or strength of creatures offered? Should I be offering rats, for the quantity, or astral creatures, for quality?

Quality matters.

If you sell the rats you'll get around %3 for a bit over 2k gold worth in rats. I think offering them would give you less.

More essence given = more karma received.

So you can offer a bunch of little things or a few strong things and it'll balance out.

I offered several astral denizens and got around 10%.. so quality definately matters.

4 Astral mobs = 1% Karma

Seems to matter a lot. When I offered Ladantine and some vampire squids I got way more than when I offered normal stuff like devil fishes.

Yes, I bash the Sea of Despair.