Hi,
In Kampala Swahili is hardly used, it'll only come in handy if talking to security guards, police, transport officers etc. If you are going to be based in the Central areas, Luganda is best. In some parts of the North or East Swahili is regularly used. I wouldn't recommend speaking Luganda to Ugandans in the Western region, though the languages can at times sound similiar the cultures are very different.
With that said, Ugandans are very welcoming and you'll find that they wish to speak with you in English. It's always nice to visit a country and to make an effort to learn some basic phrases in the popular languages spoken but English is very common here.