I think the Radio City Christmas show is a great show, and it was one that I avoided for years because I thought it would be too cheesy. I got a free ticket back in 2007 and I've gone back to see it many years since then. And it's worth noting that the Rockettes perform with a 60 piece live pit orchestra.
As for discounts, there will be discounts for Radio City anywhere and everywhere discounts are posted: Playbill, Broadway box, TKTS, people handing out discount codes in Times Sq, you name it.
As for the other shows, as long as you're not coming the week of Christmas and/or New Year's, it is often easy to get discounted tix to most shows, especially mid week, as most people don't choose to travel in mid and early December.