SwiftKey is a great keyboard. There are two really good ones out there as 3rd parties that (IMO) stand above the rest. The second being Thumb Keyboard. The thing about SwiftKey is you can't beat their prediction and the correction abilities. It's just that way and there is no wiggle room/argument there... and it's really fun to just spam the next predicted word and see what comes out of it... you really get some weird stuff going on. But, there isn't nearly any comparing to the ability to customize layout and the look of Thumb Keyboard. So, really it comes down to can you find one of SwiftKey's predefined layouts to be your look and go with its power or do you need to have the look over it which just adds a ton to how much fun it is to type because it looks cool.
And, as for stock keyboards... the problem with that (IMO - again) is that if you are moving phones, odds are you may even be moving phone companies. Go from Samsung to Moto to LG and blah blah... Each company has their own stuff, and then Verizon throws their twists into it... whatever... in the end, when I pick up a new phone, I like to be able to go in and make the thing I do most (type) be the same with that phone.
I use SwiftKey 4 with Flow enabled and I don't always flow. I am going to go download Thumb Keyboard right now and goof around with it until I fall asleep tonight I think.