The elephant in the room is Cirque du Soleil's "O". I saw Le Reve three nights after "O", and Le Reve is far better:
1) Le Reve is a more intimate stage as the audience sits around the pool 360 degrees. Le Reve had less clown time. And Le Reve had more action / acrobatics. It's a no-brainer. What's sad is we saw O on a Wed night and Le Reve on a Sat night and, despite having hundreds more seats, O was far more packed than Le Reve. Brand recognition does matter.
2) Price. O is far more expensive - outrageously expensive. Tickets are almost $200 (best Le Reve tickets are about $115). It's not worth it. There are too many other entertainment options on The Strip for a fraction of that price, like Le Reve.
Le Reve has more action, less of the extensive clown sideshow that "O" has, a smaller more intimate stage, and is just more entertaining.
My only complaint is that the theatre was too hot. Maybe they wanted to help the performers be more comfortable, but the "O" Theatre wasn't that hot.