It looks nicer (in my opinion) and is easier to maintain, also allows floating plants, and hardscape to come out of the water etc.
Downsides are a bit of evaporation (only about 1.5 litres per week in my 60 litre tank), and potential escapees!! If you have the water line too high, I find Amano shrimps will explore your living space, lol. Other thing is if you have hard water, then you need to keep that waterline looking nice using a razor weekly (when you clean your glass), otherwise you will see a calcium deposit.
With all that said, they are only small downsides, I think the advantages more than outweigh the disadvantages, but I guess it depends what you find the most important.