i think you should join the one which you feel would suite you the most rather than joining one which has more iitians !!!
eventually its all about you and your preparation and ofcourse SELF STUDY which is the most impt!!!
i think...
...you should join one which has better teachers who lay importance to understanding and application , one which has better competetive crowd , one which gives less burden on to you ie. has less number of classes in a week , one which your parents think and one which YOU THINK
according to me FIITJEE is rather better than Bansal bcause i have heard that in Bansal they lay importance to only 2-3 top batches and rest batches have 100-150 students each.
also fiitjee has good faculity , it is according to me the second best institute and also bcause they prepare you best in maths and chem specially.
but last not the least do go for a research to choose which one is better!!!