Which one is easier to get in to depends entirely on the time your application gets processed. In general, I think infantry has the most new positions hiring because its such a big trade, so that would probably be the easiest to get in to. But again, it all depends when you apply and when your application is processed. One day they may have 0 openings for 1 trade, and 5 for the other. Next day, it might be the opposite. Can't get in a trade that isn't hiring.
Each position is SO different, but for basic excitement, infantry officer probably has the most exciting opportunities.