Hello aspirant. This comparison is really difficult and varies from person to person. IIT is something which has math, physics, chemistry with different streams and institutes etc. Upsc is something that has history, geography with different opportunities for career. But in my opinion, upsc is difficult compared to that of iit. Because of

1. Competition level... in upsc many people compete and very less number of availability of posts, in IIT ofcourse many people compete but availability is more

2. Toughness .... upsc paper is really tough with vast syllabus and even sometimes out of syllabus questions, but it's not like that with IIT

So in my view upsc is higher level than IIT