Hey there,
I am sure this is a common confusion among students taking an admission to engineering college.
Frankly speaking NITs are premiere institutes in which getting into requires getting good score in JEE whereas SIT conducts its own entrance test which is comparatively easier to crack. SIT even considers JEE main score so if you didn’t perform well in that exams you have an option for SIT.
SIT is also a great place, amazing environment, great infrastructure, good facilities, etc. But when compared to NITs the faculty there is experienced and being a government institute it has different syllabus than that of a private college.
There are pros and cons but if I had a choice i would have definitely chosen NIT.