If you have the chill I'd go with the regular Rainier.

You didn't specify exactly where you are though, and it's possible chill could still be an issue for you. California has a lot of warm sunny Winter days that can really detract from the amount of effective chill a cherry tree accumulates. What I mean is that if the label says the tree requires 500 chill hours and the climate where you are gets 500, it still might not be enough. I don't know the specific situation of what your climate is like. Cherry trees often don't tend to do as well in California as they do in other states like Michigan or Washington.

I'd look nearby and see how well cherry trees are producing for other people around where you are. (If you've never seen cherries growing on a tree where you are, that could be a sign you want to go with the lower chill variety)

According to the Dave Wilson website, it looks like Royal Rainier will develop more red blush than regular Rainier, and will ripen about a week earlier, which in my opinion sounds like a pretty nice option. A small tree could probably be shaded temporarily near harvest to prevent some of the red blush if you care about color. It also says it won taste testing -- against what other types, who knows.

So, while I have not tasted it, it sure seems worth a try if you think you might be interested and can handle more red blush or installation of temporary shade.

Since you get plenty of chill hours I'd go with the regular Rainier...more yellow/less red blush means less bird attacks..still good tasting!

A cherry tree on a Mazzard rootstock is a full size tree, 20+ ft tall. You should look up a cherry training system called KGB. Otherwise, you will have way too tall a cherry tree in your yard.