I think the general opinion is that Martinez is extremely out of his depth with this amount of world class players and that there isn't actually much confidence in his abilities to take Belgium to the level they need to be.
England, whilst inexperienced have various big money PL players in their squads who have CL and PL title race experience.
Young, Stones, Walker, Henderson, Dier, Sterling, Lingard, Ali, Kane along with much of their substitutes bench have extensive experience in the Champions League, PL, FA Cup and Europa. Whilst there are probably more 'star names' in Belgium right now, there is little suggestion that they can play well as a team, and there is every suggestion that England can.