Your advisor is not always right, and they can only make suggestions. In the end, you are the one in full control of what you choose to do. So yeah, you are absolutely required to take 203, regardless. 270 counts as a flex tech, and towards the 18 credits of CS coursework (not counting the core courses). However, this fall, Prof. Bertacco is very good (and yes, I have found ratemyprofessors.com to be mostly reliable).
You can find what qualifies as flex techs here: http://eecs.umich.edu/eecs/undergraduate/computer-science/16_17_cs_eng.pdf. Almost any 300+ level STEM course will work.