Your strategy when in contests should always be based on how many people are in the contest. When you have few people in the contest, your selections should be more in line with what gives you the best chance to win. When there are a lot of people, it becomes more complicated due to the fact that if you make a popular selection, you will like find yourself tied with numerous other people.
Example:
Your contest has 1300 people
The choice with the best chance of winning is Belgium/France with France the winner. This scenario has around a 7% chance. But you may have 200 in your pool picking this scenario. So your chance of actually winning is (7/200) or .04%.
On the other hand, if you picked a huge long shot, the chance of it happening will be far worse, but it's possible you could be the only one who picked it. Something like Belgium/Slovakia (Belgium winning) only has around a 0.3% chance of happening, but that's about 8 times better than the other one when you factor in having to win a huge tiebreaker.
Of course you don't know who's picked what so you have to make a rough estimate, these are just good guidelines to follow