Synonyms for '멋지다'
I'm looking for (non-offensive) words to describe two close female friends of mine (Koreans).
Both are 멋진 to me, but they are quite different, so I feel there must be better/more precise terms.

One of them is a really fashionable and pretty-looking 언니; but also in personality, she's "stylish", outgoing and elegant. Kind of chic? The other one I can best describe as a "strong 언니", who's really confident and reliable, so I almost want to call her a cool 형.

Question

Synonyms for '멋지다'
I'm looking for (non-offensive) words to describe two close female friends of mine (Koreans). 
Both are 멋진 to me, but they are quite different, so I feel there must be better/more precise terms.

One of them is a really fashionable and pretty-looking 언니; but also in personality, she's "stylish", outgoing and elegant. Kind of chic? The other one I can best describe as a "strong 언니", who's really confident and reliable, so I almost want to call her a cool 형.

Answer

Two of your friends are fit for the word, 멋진 , according to your describing.:)
As for first 언니, i think the followings like 세련된, 옷(을) 잘 입는 ,예쁜 is fits for, too.
So how about putting them
together in the sentence when you descirbe her? for example, 
그 언니는 진짜 세련된 언니예요. 예쁘고 옷도 잘 입어요. or, 그 언니는 진짜 멋진 언니예요. 예쁘고, 옷도 잘 입고, 성격도 좋아요.

Also, you can say "스타일리시해요", using the word, stylish, in especially speaking.
this word, 스타일리시하다, which is not a standard word, but we say a lot like this, using the english word as it is.(loan language)

for example, 그 언니는 스타일리시해요.(=옷을 잘 입어요.)

I hope it helps :)

Answer

sorry! i will write again below.

Answer

First one is 맵시(있는) 언니
Second one is 당찬 언니