***** NOT A TEACHER *****
Hello, Ronao:
A teacher has already given you the answer. If you follow the rule that he mentioned, you will please everyone, including your teachers.
*****
I am replying only because I want to let you know that some (many?) native speakers do NOT always follow that rule. Please do not be surprised by that fact when you read and hear English.
1. One book * that is widely respected by learners and teachers says:
a. "When a group only has two members, we sometimes [my emphasis] use the comparative instead of the superlative."
i. His examples: "I like Betty and Maud, but I think Maud's the nicer/nicest of the two." [His emphasis]; "I'll give you the biggest/ bigger steak: I'm not hungry." (Mr. Swan is careful to say that some people feel that a superlative is "incorrect" in those sentences.)
2. Another book ** says: "[T]he natural tendency in English is to use the superlative in speaking of two."
a. His examples : "We'll see who is strongest, you or I" / "Put your best leg forward."
b. The second book is "old." But in my opinion (opinion!), his comment is still true in 2015.
Credit goes to: *Michael Swan, Practical English Usage (1995 edition), entry number (NOT page number) 138.2. / **Otto Jespersen, Essentials of English Grammar (1933), 1964 copyright by the University of Alabama Press, page 228.
