Math time!

?-4 gives 500 exp for 40 stam - that's 12.5 exp per stamina point
?-3 gives 400 exp for 30 stam - that's 13.3(333333) per stamina point

?-3 wins out there

As for not being able to 3 star since area six, are you balancing out your teams or are you using the helper set up (team 1 is strongest and it goes down from there)