I have read all the previous threads about Coat Kings but am still confused about which one to get

Ruby has always been very fluffy, but since she was spayed in November she is getting even fluffier. She went to the groomer a couple of weeks ago for a "tidy up" so looks OK at the moment. The groomer advised me to get a stripping knife, but these seem to be used more for wirey coats from everything I've read?
So I am now thinking a Coat King might be a good option, but.....do I go for the 20 blade, 12 blade? As they are fairly expensive I don't want to get the wrong one.
Any advice please??