They all allow to mix multiple languages and are pretty awesome virtual keyboards. Using them with two or more languages at the same time is a very smooth experience. Couldn't imagine to go back to a single language keyboard.
A quick overview:
- Swiftkey has probably the best prediction capabilities and with its cloud sync it has many advanced features.
- Touchpad is very close to swiftkey
- Kii is the keyboard with the most configuration possibilities. You can change really everything with this keyboard, from the size of the buttons over padding, click sounds, prediction behaviors to gestures and many, many more. It's definitely the most powerful and fastest keyboard. On par with swiftkey except the cloud sync feature. Once you you set it up to your needs your type as fast as with a blackberry either with one hand (by swiping) or with two hands (by tapping). But again swiftkey and touchpad are very powerful too and at the end it's a matter of taste.
Swype is still quite popular but doesn't have good multi language support and the experience is not that smooth compared to the other ones.