If you're the type who can't decide whether you'll have bacon or sausage with your eggs in the morning, you might have a hard time picking between the two in-game fossils when the time comes. Not to mention, the game doesn't really give you any sort of clue what Pokémon you'll get from either the Jaw Fossil or Sail Fossil.
Don't worry though, we got you covered.
Let's start with the Jaw Fossil. This fossil will produce a Pokémon called Tyrunt, which is a Rock/Dragon-type and comes with the Strong Jaw ability. Tyrunt eventually evolves into the much larger Tyrantrum but retains his Rock/Dragon-type form.
The Sail Fossil produces a Pokémon called Amaura. Amaura is a Rock/Ice-type Pokémon, and comes with the Refrigerate ability, which turns any Normal-type moves into Ice-type moves. Amaura evolves into Aurorus and retains its type, but gets much larger, much like Tyrunt and Tyrantrum.
So there you go. Either end up with a sweet looking T-rex or a majestic dinosaur. Either way, there is no wrong choice here.