Whoa there folks! Surely you can see that, as usual, the best answer is, it depends.
If you are happy with the nature of RN life, maybe have family or friends in the Service, or were a Sea Cadet, or are simply convinced or unperturbed by what naval life will hold, then join the UAS and check out whether flying is for you. Enhance your CV at AIB by having a few hours under your belt and be able to talk about your passion for flying from a position of a whole 10 hours experience!
If, on the other hand, you know that you love flying, you've had a few flying lessons etc, but you are unsure that you will like bobbing about on the oggin and other aspects of naval life, then try out the URNU.
Don't get hung up on the idea that there is some scoring system at OASC or AIB whereby you can get black marks or epic scores for individual choices you have made. There are very many 'ticks' you can get at these selection interviews, and even more ways of gaining them. Nothing other than 'holds views incompatible with military service' is going to give you a killer fail. You might get 'two ticks' rather than 'one tick' at AIB for being a member of the URNU rather than the UAS. However, if the UAS was the place for you, then you will pick up that extra point, maybe even more because you are able to convince them of your passion for flying