If you're only planning on getting one Tiffany piece, I would consider something that's been around longer, or that could be more versatile to go with more looks. I'm thinking of the bean, DBTY in SS, or a key, for instance. I tend to think that the pieces that don't shout Tiffany (not plastered with the brand in huge letters) are a bit 'older', and age better.
That being said, if you're going to get one of the chunky choker-style necklaces, I think the toggle one is the best choice by far.