Which is better in terms of placements, SCT Trivandrum or a private college like Rajagiri, FISAT or SCMS?

It entirely depends on the branch that you are choosing. If you go for CSE then all the colleges SCT, RAJAGIRI, FISAT, SCMS have good placements opportunities. But in the case of other branches, Kerala colleges have a low level of placement except for NITs and IITs.

And everything depends on you how much effort you will give to your course so that you can obtain experience as well as knowledge so that you can get a good job apart from your college placements