I'm sure that 'the exigencies of the Service' will trump all else, though there must be many fewer alternatives today than when I graduated from ANS in 1965. When our Course Commander came back from his visit to London to announce postings, there was enormous surprise to hear that two of our number were going to Javelins. No Navs had gone that way in ages - but the post-UDI deployment to Zambia had changed everything. As things turned out, events changed things again and they ended up on very different types. At that time, I think we all were able to express a type preference, and the chap with top marks got what he wanted, provided there was a slot available.
Going a little further back, and I stand to be corrected on this, but I seem to recall that officers in the GD Branch could expect to follow a 'balanced career' during which changes in role would be considered as entirely normal. 'Fast Jets' and the like did not exist at that time and, having graduated to Wings standard, the expectation was that you should be capable of operating in all current roles.