Mate,
If you are successful then you have a good chance of going crewman at the present time. But as previously stated the RAF will put you were it wants you. From my angle being a WSOp EW or AEOp is the longer course therefore more chance to re-stream if you fancy it. Also the course before the streaming phase is short so purposely failing somethuing to try and edge in another direction is only doing your chances harm. PS the chaps at 55 are really great although as a student you may look from a different light if you have a family reason per say to go south and there are other candidates who are as good as you then just speak to them they wont bite as I am sure they will tell you.
GOOD LUCK