Its almost bound to be Marines of power armoured nuggets of some breed (not sisters though). Its the biggest seller GW has, and its the easiest army for new players to pick up on, who would be the ones buying the boxed set. Space Marines are *everywhere*, in books, games, and "movies".
40K differs from Fantasy in the sense that Fantasy doesn't really have "the newbie army" - in 40K most players start with marines, then break off and play a different race. So it makes sense for GW to put them in every starter set. Fantasy doesn't have one of these "newbie" races - there's no race that generally dominated by newer players, which is why they're free to cycle the races that show up in the starter set (I still have my brettonians vs lizardmen kicking around somewhere).
It'll be marines of one breed or another, I can promise you that!