For a 20-25G you are right in the range that can be served by all the main filter types, sponge, internal, HOB or cannister and OM47 has laid out a nice set of pros and cons to help you with your decision. Just wanted to toss in a post to agree. A small external cannister such as the XP1 mentioned or a nice Eheim would certainly be the most quiet, deluxe way to go, but the others all have their advantages too. For a display tank, I also like the option of an inline heater that's spliced in to the external filter return hose, so that its not seen in the display tank and so that its heat is evenly distributed. Having larger amounts of media volume in your filtration is always a good thing.
~~waterdrop~~