Answer:
1. Betta Fish
2. Yes. Choose a terrarium that has at least 5 gallons of space per 2 crabs.
3. They require companionship! Hermit crabs, despite their name, are social creatures that thrive best in groups of two or more.
4. Both are entertaining, but I prefer betta. Hermit crabs, in my experience, are extremely sensitive and difficult to keep alive. Hermit crabs proved to be much more difficult than I anticipated, but bettas are a little more straightforward. That is just my opinion; I am sure you will do fantastic with whatever you choose.
I hope this helps you
:)