ok so I guess im probably way too late for advice but I spent a year in SF and in that time developed a healthy addiction to sushi.

First off, union square is possibly one of the most tourist areas of SF so any sushi that you find in the area will be the kind that rides around the restaurant in a little boat and will not be any good.

I agree that Zushi Puzzle is one of the best places to go. They have really fresh fish and great rolls.

The freshest fish in San Francisco is at Kabuto on Geary Blvd between 15th and 16th ave. This place is spectacular but it would be really far from you as it is nowhere near downtown.

Another place that I liked a lot when I was there was called Ace Wasabi in the marina district(very close to downtown and also a cool place to visit for the nightlife). Its one of the more trendy sushi bars where all of the young folk hang out before they go get drunk at the many marina bars. They had more americanized rolls but what they served was still a lot better than anything I got outside of california.

Also, for a really good lunch you should go to Pacific Catch (also in the Marina). Be sure to order the tuna poke, it is the best poke that I have ever tasted and the one thing that I still crave the most after moving away.