For me, when I lived in Kenya English was the lingua franca for official/business activity. Some older professional Kenyans I knew learned Swahili only after learning English (and their mother language, Luo for instance). On the street, Swahili was standard.
The Tanzanians I met however, considered Nairobi Swahili not even 'real' Swahili and more of a mish-mash for disparate speakers who might otherwise choose their mother tongue when possible.
And then there's Sheng - a creole that is an even further evolution from Swahili and various other languages.