Hey Frank;
I'm no real expert on sake but appreciate a good one when I eat sushi. So this much I know:
The more polished the rice is, the higher quality sake it produces.
Of the different grades of sake I believe it goes like this:
Best - Junmai Daiginjo
Very good - Junmai Ginjo
Good - Junmai
I also have had very good sake without the Junmai precursor; however, what, why, how they're different, I don't know. I know we can Google it but trying to do this from memory so you might wanna verify before you walk into a sushi bar and order some. However, pretty sure if you order anything with a Ginjo grade or better you won't go wrong.
Cold or warm? Typically if you order warm sake it'll be a lower grade one. I used to only drink warm sake until I went to Japan and had cold Morita sake. I was working for Sony at that time and at the opening dinner of the line-up meetings they did the whole bust the wooden cask and pour it into a glass that was inside a square wooden cup (...you drank from the corner until it was gone and then you drank from your glass...) ceremonial thing. It was the first time I had cold sake and it was really good. However, even with that said, when it's cold here (...when the temp is 65 degrees or less...), I'll order warm sake to take the chill out. Yeah, yeah, coldness is all relative.
A bit of caution: sake is like tequila...it'll sneak up on you. You'll be drinking, eating, talking, drinking, laughing, eating, drinking, talking, drinking and then you'll have to stand up to go to the bathroom and then WHOA! Buzzed out. So go easy at first and enjoy the food with a little bit of drink and ease yourself into the world of sake.
Hope this helps a little and when you do finally try it, lemme know how it went.