Obviously, it's better to opt for IIT over BITS. After all, it's one of the best engineering institutes in India at present and for all time. Infrastructure wise BITS even is much better than most of the IITs. The BITS Pilani has a splendid campus spread over 990 acres for all the departments. All facilities are provided here are somehow better than IITs. As fees structure difference is there. But still IIT is the most prestigious institution. In term of Placement IITs are the best in India undoubtedly. If you consider the average package offered of all the IITs, it's better than one single BITS. If you get seat in any of the IITs through JEE advanced you must be suggested to go for it. It's a huge opportunity.

Hope you have understood.