I don't think we will be restricting our users to a certain battery pack, so you guys are just creating an option for them
So the idea is, you choose whichever you think is the best, but try to take into account these things -
Price - try to keep it low cost
Availability - use things found easily all over the world, or give information about alternative components which are sold in other parts of the world
Danger - include warnings... if you are using Lithium batteries especially!
Design - try to make your module noob-proof. Use connectors that can't be inserted the wrong way, or design them so that incorrect connection doesn't do any harm.
Flexibility - Make your module flexible... Could they use a 6V battery? If not... do we need to design another module for this?