It makes little difference which way you get your entry visa. Although for the sake of peace of mind, and simplifying the entry process I recommend getting your visa done online. You will need your passport, two passport sizes photos, and yellow fever vaccination.
I have heard talk of Uganda requiring everyone to get a Visa online before entry but as of this writing (Dec. 2017) you still have both options available to you. In summary, entry is easy, get your Visa either online (highly recommended) or at the airport upon arrival.