So I got to try on the Tiffany T smile necklace in store last week. Unfortunately I did not take photos, but it never crossed my mind because it was so underwhelming to me.

I too was looking for a minimalist necklace but that T smile in the XS is really small.

And for some reason, the diamonds did not sparkle very much. I even tried on the size Small and Medium, and still the diamonds did not sparkle. Maybe it was the lighting, but when I tried on diamond-by-the-yard, that diamond glimmered beautifully!
The Cartier love necklace, is pretty. However, I do see a lot of similar replicas and/or fakes.
And I personally do not like it when a chain is wrapped around or looped through a piece, because I don't like metal rubbing against metal.
But that's just me lol, if those things don't bother you, then that's good!
Chains for Tiffany necklaces are very fine and dainty. They will get tangled if you don't store them properly. Cartier chains are a bit bigger, and I would assume, sturdier, but the chain will stand out just a bit more.
Hope that helps and good luck! Happy birthday!