Rent the Runway vs. J.Crew

My work holiday party is next Friday. Yes, we're celebrating it in the New Year instead of at Christmas time. I think it's time I use Rent the Runway. I've been meaning rent a dress since I signed up in November 2009. I've always come up with an excuse, like wear something I already have, or buy a dress, instead.


Here's what I'm thinking of so far and I need your help. What should I choose?

HALSTON HERITAGE 

Confetti Dot Dress

Rental $85 Retail $595

Source: renttherunway.com via Emily on Pinterest

KATE SPADE NEW YORK

Mademoiselle Dress

• Rental $50
• Retail $395

Source: renttherunway.com via Emily on Pinterest

Or should I wear my J.Crew's Silk chiffon Sophia dress, hanging in the closet? I have this same dress but with tiny white polka dots.

Source: jcrew.com via Emily on Pinterest