By default Congo would be better off. None of the other major figures in Congo of that timeframe were anywhere near as kleptocratic as Mobutu was. The key question is how Tshombe gets to power. He was dismissed by President Kasa-Vubu in October 1965, and in November of that year Mobutu staged a successful coup that resulted in him staying in power until 1997.
Tshombe's power was based on the support of provincial political bosses, customary chiefs and foreign financial interests. There is the potential for him to take power through another Congo Crisis: if Tshombe responds to being fired by Kasa-Vubu in the same way Lumumba did, by addressing parliament and having Kasa-Vubu fired, you get such a crisis. Tshombe then needs US supports, which involves Mobutu winding up dead somehow so US support won't go to him instead. If he is halfway competent, he could get Congo on the level of Kenya or Congo-Brazzaville, which will have butterflies across sub-Saharan Africa.