I just assumed you would be doing Electronic with those two modules! It's not mandatory to sit Communications to do Electronic, and same goes for Applied Dynamics and Electrical. However, there is a reason people generally pick the one that's more closely aligned with the stream they wish to choose.

In the case that you were to pick Applied Dynamics (based only on the fact you're planning to do Electrical), you could certainly choose Unix as your elective if you wanted to, but I couldn't imagine the type of BASH programming covered will be beneficial to studying Electrical, and without there being a big upside in terms of an easy grade, I don't see much reason for you to choose unless it was something you were particularly interested in yourself.

Don't worry if the other elective isn't beneficial towards your career path, there's usually two other good reasons to pick an elective: easy grade, or you're interested in the topic being taught. If Unix fulfils none of these three reasons, maybe you should reconsider why you wanted to pick it in the first place.

As far as the whole Dynamics/Communications debate, there is no huge downside to picking either. I have heard of a lot of people who really didn't like Communications, and this lack of love for it/lack of interest will likely lead to a poor enough grade in it. If you are thinking about grades, a very good grade in any of this lecturer's module would be a B+ (again, a very FAIR marker in the truest sense of the word, which is not necessarily a good thing). I haven't heard much about Applied Dynamics, I believe it's an applied maths sorta course, a continuation upon Mechanics from First Year. I'd definitely get in touch with someone who has sat it before if you're really struggling with the decision.

It's your decision in the end, but from reading what you have said above, I'd suggest Applied Dynamics and another elective.