While neither would be my teen's style, I think the open heart one is TOO timeless - it's a bit fusty for a teen nowadays, surely?
The blue double heart tag feels much cooler and more modern.
You absolutely know her best, so is Tiffany her style? Instantly recognisable classic/traditional brand name? Because you're paying a premium of about £280 for the brand name ONLY so you need to be sure she's not going to just silently slide it into a drawer and never speak of it again!
Some teens might prefer something a little cooler from an independent creative like:
www.wolfandbadger.com/uk/designers/cartergore/
www.wolfandbadger.com/uk/love-u-happy-face-pendant-gold/