Definitely check out Sage. Been there twice and had two great meals.
I also have been to American Fish once and it was good, but I would much rather go to Sage. Had a lobster wellington, which is about the same price as the lobster pot pie at Michael Mina and the lobster pot pie is definitely better. If you have a chance, go to Michael Mina in the Bellagio and get it.
I have also been intrigued with Sirio, every time I walk by the place it is packed.
We ate at this restaurant on Saturday, and we found it very, very expensive for average fish. The star of the evening was the cornmeal crusted trout. My Branzino (3 small medallions) was tender, buttery and rather bland. My partner requested no sauces on the plate, and it was still served with three swipes of colorful sauce streaks under the fish entree. The plate was taken back and the fish was moved away from the sauce on a fresh half plate but the small salad and 3-tiny bacon wrapped potatoes were now sitting in the sauce on the other half. It may have been a communication error, but it was disappointing. Two fish dinners, no alcohol, one soft drink and the bill was close to $100. Very expensive. The restaurant was almost empty. Atmosphere was nice. The next time we are at Aria, we will try Sage.
I highly recommend Sage. Great food (familiar but with a twist), service was casual yet attentive, and the dining room is what I'd think "dark romantic" (if you're into plush carpets, dark velvet, beautiful chandeliers, and walls with large purple projections of Édouard Manet's "Le Déjeuner sur l'Herbe (The Luncheon on the Grass)).
My recent experience found here: http://chowhound.chow.com/topics/781843