The difference between 执著 and 固执 I had this sentence: 麦当劳一直执著地坚持着浓浓的美式风格。

I would translate '执著地坚持着‘ as 'stubbornly persist in'

I was told, '执着‘ has a positive sense, whereas '固执‘ has more of a negative sense of 'persisting even though you know it is wrong to do so'.

Would you agree?
He/She is right. 执着 is more positive,like "persistent" or "Never give up,never surrender!"
While 固执 stands for "stubborn",you can use it to describe annoying kids who just refuse to eat carrots...something like that.
 However,sometimes (less commonly) 固执 can have positive meaning too. It really depends on the situation.