The Apple iPhone 6 already has incorporated in it the QuickType virtual keyboard and is said to be a really good improvement on its previous virtual keyboards of earlier iPhones. One of its biggest assets is its ability to know that you are sending a formal email and would thus automatically ensure that you do not use inappropriate format, such as typing a slang word. This is really beneficial, considering one would not want to get into the bad books of one's superior at work by sending absurd abbreviated words which he / she would not fathom or, even worse, some horrid slang! QuickType also does give you word prompts based on your message so that it is quicker to enter your response.
Since now 3rd party keyboard apps can be used on Apple iPhones, you have 2 further options for the iPhone 6: SwiftKey and Swype. While the latter had an initial promotional offer at 99cents, the former is free. Those who have used & reviewed both these apps seem to prefer SwiftKey because it seems it is a quicker learner of your vocabulary! It also provides an optional cloud service which is totally free for backup & sync of your data. This data is secure, in an encrypted form, as stated by the company. One disadvantage that users have noted is the inability to leave a space after a punctuation, so those who are sticklers for perfect English writing, are going to find this downright annoying!
In comparision to SwiftKey, Swype takes time to get "trained" to the words you regularly use. Its unique advantage is that if at any time you wish to use numericals, you do not need to swap alphabets to numbers, but just press down for an extra few seconds on the alphabets & voila - you have the numbers on your virtual keyboard. Though like SwiftKey it has a cloud service for storing words & can even sync across varied devices, this is not currently enabled on the iOS app, but stores the data within your own Apple device akin to QuickType.
Regarding Emojis: note that all 3, QuickType, SwiftKey and Swype, all have emojis too incorporated. So you can send smiley or other cute images with messages to whomsoever you are communicating with.
I would suggest you try out all 3 for equal duration of time before focusing on the one with which your are most comfortable.
Check out also this Ask Expert page: Searching for keyboard for iPhone 6S