+Fastest for Japanese input
+Switching languages is easy
Google Japanese input
+Almost as fast
-English support is poor - but you can switch input methods
+Still excellent for handwriting input - possibly useful for beginners to Japanese
-Considerably slower for Japanese
So, every once and a while, I look through the Google play store and try and find a better way of typing Japanese on my mobile, because frankly, I do a lot of that, and a couple of minutes investigating which method is fastest will pay off in the long run.
Previously, I compared the Swype, Google, and Wnn keyboards. Since then, I've changed phones twice (once to an Aquos Crystal, then to a Kyocera KC01). The increased processing power of these more modern devices makes switching keyboard methods easier, so for a long time I was switching between the Google methods, which still provide excellent predictions for both languages*.
More recently I have been using the Swiftkey keyboard, and despite the fact they have done away with the 12-key keyboard (which works really well with Japanese input), I have increasingly found that it is quicker than Google Japanese Input.
To be sure, I conducted a small test using the following text:
I should probably have used a shorter text... My thumb aches... But this should hopefully make differences between the different methods more apparent.
So, this is how long it took to type with each method:
6:06 with SwiftKey
6:20 with Google Japanese Input in 12-key format
7:49 with Swype keyboard in qwerty format, tapping, not swyping.
These results tally fairly well with my more qualitative observations; Swiftkey is only slightly better at Japanese than Google, but both of these methods are superior to Swype. I would point out that swyping is still not particularly usable for Japanese, but the word prediction is just about sufficient to make tapping viable.
As well as being the fastest method for inputting Japanese, switching to English in SwiftKey is faster than changing input methods with Google (again, footnote*), and the English prediction for SwiftKey is fairly capable. Nonetheless, for words with unusual characters, my gut feeling is that Google performs much better. Additionally, my main gripe with Swiftkey is that it is very easy to mis-press the vowel extension "ー" key instead of "a", and SwiftKey's programming isn't intelligent enough to reinterperet the extension as an "a" depending upon the rest of the word. Then of course, Swype has an excellent integrated handwriting recognition system, which might be worth it for those who need to input characters they don't know the pronounciation for (I imagine, however, that this is mostly going to be in the context of looking words up in a dictionary, for which aedict offers a fairly good method that isn't keyboard-dependent. Plus, free handwriting alternatives now exist).
The bottom line, however, is that SwiftKey is faster and more convenient when switching languages, and for me, these are the deciding factors.
*As I've mentioned before, despite being able to type English using "Google Japanese Input", the prediction is awful, and so I rely on changing input methods.