IIT

For FIITJEE AITS, the paper is sometimes 60% out of syllabus and the ranks predicted by them are sometimes very demoralizing. My average JEE Mains score in FIITJEE AITS was 50/360, while I got 169/360 in the actual exam.
For Resonance, tests paper is a bit tough than the actual one, but is properly set and considering the pressure in competitive exams, it almost accurately predicts your final score (still not rank).
So, in my opinion, you should consider following things in mind before choosing a test series (in descending order of importance):
Level of Difficulty: Not easy and not very difficult (a bit difficult is perfect).
Solutions provided by the institute.
Time of Exam: The time period should exactly match the actual exam.
Number of people giving tests: That is, the popularity of test series.
Detailed analysis of your answers.
In my opinion, you should prefer Resonance or Aakash over FIITJEE AITS.

Smart practice is the key to crack IIT examination. Don't forget that cracking IIT exam is tough and you need to score 300+ marks. This is impossible without smart practice. Let me share some of the personal tips and tricks that I followed to get into IIT which may also help you.
1. Understand what works best for you
You need to understand what is the best schedule "you can follow" and not the one you force on yourself. Find out the best time that suits you or which place in the house you are comfortable in. The more concentrated you are when you study, better will be your preparation. It may sound silly to some, but this has helped me prepare faster and better.
2. Work on your weak areas
Everyone has a strong subject which they understand easily and can solve questions correctly without worry. The problem is with the weak areas. I myself was not good in chemistry subject. Then, I started working on it more and made it the primary focus of my studies. Within no time, I could easily convert it into one of my strong suits. So, make your study timetable that you are comfortable with and allocate more time to weak topics.
3. Practice as much as possible
Practice is the backbone of your path to rank in IIT. It is the most crucial element. Most of the students today focus only on studying i.e. reading the chapters and reading the illustrations and scribbling on the pages saying that they practice. When the real exam happens, these students say The paper was very difficult or they did not get much time.
For example, a pilot cannot learn to fly by reading the books. They spend a lot of time practising. Practice is the most important point that you need to focus on in case you are really serious about cracking the IIT examination. For practice, I personally used the CareerOrbits IIT-JEE course and would recommend the same to you too. Their questions from the CareerOrbits courses are far better than the branded institutes and cannot match their quality.
4. Revise repeatedly all the incorrect attempted questions
Always revise each and every incorrectly attempted question repeatedly. If you miss this, you do not get the rank you want. By practising repeatedly, your brain automatically grasps the concept and after a certain point, you can solve questions with ease. And yes, it was easy to do so for me since the JEE Advanced course from CareerOrbits tracked all the questions that I attempted wrongly and compiled 3 revision lists for me.