Depends how much you value your fish. If you faithfully quarantine every single thing, including corals, that go into your display tank and maintain the highest bio-security like using different nets between tanks there would really be no need for a UV on your display tank.
If you do not practise strict quarantine I would suggest a UV if you really want to safeguard your fish. Efficiency of a UV depends on factors like the contact time, the bulb life (ideally change every 6 months), turbidity of your wate (best to pre-filter the water before running through UV), flow-rate of the water. If the water going through your UV is too fast, its effectiveness would not be there. Personally the higher the turnover rate of water through the UV, the higher the effectiveness of the UV reducing pathogen load in your water. If you had been to CoralFarm you do notice how big their UV units are compared to aquarium-sized units.
If you want a UV to really work, there's no harm in over-sizing the unit as this would help offset the drop in UV lamp efficiency over time. If you are not worried about electricity bills I would suggest a 36W unit at minimum, 55W would be optimal IMO. A 55W unit should be able to handle your entire return flow-rate of a 20 gallon and still function optimally. If you do not have a quarantine tank, you can utilize your sump for quarantine and run your return pump through the UV prior to entering your main tank.
I do have a 55W unit from Laguna about 4ft long which I used. I'm about to throw it as I cracked the glass sleeve inside and the only way to get replacement is to order from overseas. If you do not mind sourcing for the replacement part I could give the UV to you.