should i prepare for sliding?
Ask with your seniors if you are
eligible to attend the same interviews as ECE folks. If you have the eligibilty, just keep track of what ECE guys are studying (many times, only 1 subject will be different per semester, or the examples/context of the concepts used will be different). If not, you should prepare for sliding. If you are indeed eligible then you should not worry.
In terms of knowledge; it depends. Communications will focus a lot on wiring/transmission means and wireless data transfer etc. - this is very useful in telecom industry. Instrumentation is basically measurement, testing, power calculations, performance tests, etc. (you can say lab work). This is useful in process control industries and in semiconductor fabs/embedded system hardware testing.
For your particular purpose I should say it wouldn't make a difference whether you study ECE or EIE. Either way you will be able to cover up whatever's missing between the two courses with some efforts.
Also, I do think ECE is more mathematical than EIE due to the nature of the subjects. This should not be a concern practically. If you are interested in further studies, both ECE and EIE are eligible for further studies in both branches.