Under the current election system, Farage has almost no chance of becoming PM, Slough UKIP achieved a sizeable proportion of the popular vote. Corbyn has some chance. Neither would be my choice for PM, but as we live in a democracy, I'd have to live with it. Saying which would be worse is difficult because UKIP as a single issue party again doesn't have a credible set of policies or the wherewithal to put them into place.
Out of the current electable parties, my opinion is that Corbyn would be my least favourite, and probably the worse for the overall economy as left wing economies tend to perform less well that right wing ones.