How Effective Is "Rosetta Code" as a Teaching Technique?
Here I think the teacher's instinct and experience is the best judgement. You know your own students best. Your worry, of course, is well founded, especially since the two languages are so different. 

Learning to think functionally is a big deal and a mind expander. In some ways doing it cold-turkey (with no support) is good to do, since the students need to just thrash about until the see the A-Ha. But that can be very frustrating. It isn't good to leave your students frustrated, of course. 

Another reason for not using the technique you suggest is that you don't want their functional code to be just misspelled C#, using ideas from imperative and/or object-oriented thought patterns. Code in one language based on knowledge of another can be truly ugly and the programmer can miss the elegance of the new language. 

And functional program can be truly elegant and mind expanding. 

To balance the trade-off you can, of course, use "Rosetta Coding" only sparingly. And only when it seems really needed. Perhaps you could start out without it at all and use it only in situations in which the students are struggling, rather than as a main-line technique. You could also use it only when working with individuals (office hours) rather than with the larger group if a student needs to be reminded of the basic structure of an algorithm. Personally, I wouldn't use it as the main structure of the course and am happy to let my students thrash a bit in mildly turbulent waters. But that needs limits also, of course. 

Salt is fine, but don't over-salt. Judgment and a bit of restraint, perhaps. 



Since the two languages are from different paradigms the difference in resulting code can be quite stark. For example, reversing a list in linear time in Scheme is quite a lot different than you would likely program it in C# with mutable data. But even Java and Ruby, closely aligned, are not the same language and have different thought  patterns. It is the thought patterns that are the most important lessons of the various paradigms.