I have the standard PowerBlocks and I love them. I purchased the 40 lb KettleBlock a couple of months ago. It seems as sturdy as my other PowerBlocks.
I think the better purchase would be the 40lb KettleBlock as the 20lb is just too light and you will grow out of it quickly.
While they say you can do any exercise you can do with a standard kettlebell with a KettleBlock I’m not so sure. The shape is different. I think certain exercises like kettlebell Swings would be fine but I think the kettlebell Clean might be uncomfortable because the KettleBlock doesn’t have rounded edges like a standard kettlebell does. Your forearms will probably get a little beaten up.
The handle is a bit differently shaped as well. It’s fine for me as a woman with small hands but I don’t know how it might be for someone with larger hands.
This is a good purchase for space saving.
You probably only need one to start with.