IMHo, statically, the IRS is the best.
I owned all three chassis (!!) and the IRS is the best, most technically sound.
The extreme rc racing version is of very good quality, but the cells are not closer to the shaft: a big preferred mod among tc3 users.
It is also a little wide to my liking. The overall build quality is excellent though and very nice to look at.
The BMI, and i am sorry to say so because i know a lot of ppl love it has 2 major flaws imho : The chassis posts flex under stress because of their shape,
and the motor mount has the motor sit higher or lower depending on the pinion used, which is just weird...
plus mine has the wrong bearing diameter recessed in the motor mount so i had to modify it otherwise the transmission would bind
Of course, you mileage may vary and i m sure a lot of ppl are quite happy and succesful with either one of them, but this is my own experience.