Which language to learn?

I think our point is that most people in Kampala don't understand and don't speak Swahili. (And I would hazard that those that can speak Swahili can probably speak Luganda as well, even if they weren't of the Baganda tribe.) So I'm not sure how you can "communicate and get along with locals in a better way" in a language they don't understand.

In Uganda, I never assume anyone can speak Swahili unless I hear them speak it first. In Kampala, I sometimes switch to Swahili with boda drivers but that's only if they try to charge me an unreasonable amount for a ride. That's when I pull out my 'I'm not some dumb mzungu' shctick. Maybe half of them can understand Swahili.

There are higher numbers of Swahili speakers on the Eastern side though. Busia definitely. I heard people speaking Swahili in Mbale. I spent a weekend at Sipi Falls last year. Seems everyone there speaks Swahili. The people in that area are related to the Kalenjins of Kenya.

On the Western side, I can only remember having a couple of conversations in Swahili. Once with the guy selling tickets at the Bismarkan counter at Kabale. And once with the people operating the taxi from Kisoro to Kabale. But again, I don't actively look for people to speak Swahili with in Ug.

1. Wasuze otya! Luganda - widely spoken in the country, understood throughout and most of your time is going to be spent in Uganda, so it's a no-brainer.
2. People do speak English, but speaking Luganda will be a big plus for getting closer.
3. Swahili was used widely as the lingua franca of the armed forces under Amin and therefore widely stigmatised in Uganda.
4. If travelling out of Uganda - i.e. in the other E.African states, you can learn some Swahili a few weeks before heading off. There are many common words and structures between the languages.