Arwen grew up hearing about her great-great-grandmother, about her beauty, her love, her courage and her sacrifice.  She also grew up hearing about how similar they were, how alike they were in their beauty and their smiles.

What no one ever talked about was how different she and Luthien were.

Where Arwen was gentle and serene, Luthien was passionate and stubborn. Luthien loved freely, laughed even more freely and she knew how to weave a spell as well as she knew how to dance. Arwen loved deeply but she was always careful with her heart, her family's history whispering in the back of her head (Peredhils are made for tragedy) and her laughs were softer than Luthien's (who's were loud and boisterous) but they were no less genuine for that. In that way, too, they were alike. Their laughs always had real warmth in them.

Luthien could weave a spell, flawless and perfect, her mother's blood running true, but the only thing Arwen ever learned to weave was cloth. It was never in Arwen's nature to wield a sword or perform loud, grand heroics. She preferred the quieter things, like making sure there was enough provisions for a siege, exact and precise planning and stitching wounds close with the same sharp precision she used to sow banners.

Luthien lived hard and fast, a burning inferno of light and power. She walked tall and confident because she had been raised under the stars, secure in her mother's Girdle and her's father protection; it never truly occurred to Luthien to be afraid of anything. 

(She knew fear, of course, but never the gut wrenching terror most would have felt walking freely into Angbard in order to wrestle a Silmarill from Morgoth's crown)

Arwen never had a Girdle to protect her. And while Imladris was a hidden and protected realm (and Lothlorien a secured one), the mortal realms of the North were not.

Arwen was always more aware than Luthien of the nature of the world because she watched as her mortal friends and their realms left her far too soon (it was always far too soon).

But the way they differed most was in their knowledge. Arwen could be patient, could respect and abide by her father's wishes because she knew in truth that it was always going to be her choice. No matter what her father desired, the choice-to be Elf or Man, to love or remain-was always hers.

Luthien had no such reassurance.