What normally happens is that all the regimental reps at RMAS throw their car keys into a big bucket and then invite the most attractive looking OCdts to fish out a set. After that, well, it's all between consenting adults...
Alternatively, they will be looking at you to have thought through the options and come to a rational decision, based on factors that you can defend. Professional aptitude; family connection; infantile infatuation etc etc. All very valid in their own way. If you think about it, the army has selected you to go to RMAS because it wants a rational decision maker who can then execute his or her decisions, and take other people along at the same time.
So, if I were you, I would find out as much as you think you need to know about the regiments and corps you are interested in, and what a YO's life is likely to be like, weigh up the pros and cons, and then apply.
I'm planning on going to Sandhurst at some point in the next couple of years, and I'm at a UOTC, which means that I've been able to get a fair bit of information about different Arms. Right now I'm thinking either the Royal Engineers, or the Royal Artillery, as both are interesting to me, so what I'm thinking of doing is contacting the local TA unit's for each of those arms, and seeing if I can't come along sometimes and get a better idea of what they're about.