For whatever reason my neighbor seems to think my opinion matters and wants to know if she should plant regular Knockout or Double Knockout. I have regular and it is OK, great the first couple of years now needs to be cut waaaaay back, I have Pink Double and I like it better. My favorite of the Knockout family is actually Home Run. Does anyone have any thoughts on these?
Our soil is terrible down here and I am actually taking out a Belinda's Dream for non performance the last 2 years and I outright killed a yellow Knockout.
By the way I am thinking of replacing Belinda with M Carriers. Thoughts?

I find the Double Knockouts to be more attractive and rose-like, also they have proven equally tough as the originals for me.

I have double pink knockouts Was gorgeous last year.....for a few weeks, then just so-so for the rest of the year. They like to grow way bigger than the plant label says. Up to 8' tall here so am considering finding a replacement that is shorter and provides some benefit to wildlife.

Same experience with mine too!

Size is one issue I have with knockouts. The other is that they are so full of thorns. I got so tired of having to prune them and looking like I had been in a cat fight, that I took them all out. I did grow Home Run and I agree that it is very pretty. This is Home Run taking over the patio.

Wow @sall20 ! Is that one Home Run? Mine gets nowhere near that big.
I spent today pruning the few roses I have back, including pruning regular and double pink knockout waaaay back. I want to encourage something from the roots. There were a lot of old thick canes that I took all the way down. If they don't do well then I will just replace them with new smaller ones.

Yes, that is Home Run. You mentioned old thick canes. That is the other issue I had with Knock Out roses. If you don't keep cutting them way back, like almost to the ground,they get those ugly canes. And then, we're back to those thorns! ( I guess it's obvious by now that I'm not a fan of these roses.)