Since you will only have one or a few fish in there at any given time (I hope) you will not need the bigger one. The small one will work well enough. It's actually a good filter.
If you have to, you can even adjust the flow by moving the intake tube from the left to the right, which will slow the water down. This will be useful if you have a sick fish that is having trouble swimming.
You can also boost the biological filtration ability of AquaClear filters by filling the chamber over the sponge with filter floss, which will increase the places for beneficial bacteria to cling to.