Traditionally (before the -901 series exams were rolled out), the order was

BSCI

BCMSN

BCRAN

CIT

with the thinking that material in BSCI & BCMNS built on the foundation laid in the CCNA curriculum. Now the BSCI and BCMSN exams have been updated; BCRAN is replaced with ICSW and CIT with ONT.

I've only taken the -801 series BSCI so I couldn't tell you how different the current exams are.

BSCI suuuuuuure ain't easy.