There is no smartthings integration with HomeKit at this time, and none as yet planned for the future. HomeKit requires that every device have a special hardware chip in it for security, so it isn’t something that can just be done through the cloud. And devices cannot be retrofit to be HomeKit compatible, which is why Phillip said to come out with a new hue bridge last fall. More details are given in the following thread (this is a clickable link)
There are some devices which will work with both HomeKit and SmartThings, but in the UK that is currently limited to the Phillips hue bridge (generation two).
In the last eight months I have moved most of my mission-critical home automation to HomeKit – compatible devices because I needed the reliability. But it is a very limited feature set compared to SmartThings. And a very limited set of devices so far.
I like HomeKit very much for the basics, particularly control of lighting, but there are very few device choices right now.
If you’re going to use the hue bridge anyway, and you already have an iOS phone and an iPad, you’re all set for HomeKit. And you can continue to use the same Hue bridge with SmartThings as well if you like.