CAS is ranked higher overall as a school and does have a really strong CS program. However, CAS grants BAs in CS, whereas Tandon grants BSs, and that may make a difference to some employers. Also, in CAS you have to go through the CAS liberal arts core, and the Tandon core is different and more STEM based. Tandon is also in Brooklyn, is newer/less-established, and has a fully different campus. I’m not in CS but I am at CAS and I will say that all my friends in CAS CS have had a great time and have good career opportunities.
If you also want to study math, the CAS math department is also super great, so you could minor in math (you could cross-school minor from Tandon but bc the campuses are so far away it can make your schedule hard) or (possibly) double major.