Do you have a photo?

There isn't a real advantage or disadvantage, but a square or rectangular vanity sinks fit in more with contemporary style.

We ended up going with round sinks. Our home decor is more traditional than contemporary. The bathroom is still a work in progress but here it is so far. Thank you for your feedback. Very much appreciated!

I recently debated this question in my head. Wanted rectangle but ended up with oval. Just picked them up today. Brand name is Kallista, kohler makes them.
I think in the end I didn't like how the rectangle ones swooped down, I preferred how the oval was rounded all around down to the drain.
I went with very classic traditional faucets so I think that went with the round or oval better as well.

It is a design preference. Both work well and there are a large number of models in both round and square. There are all sorts of basin shapes: bowl curiving down, flat-ish etc... You just need to find the one you like