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I think that the fact on google one form of a word is ten time more common than the other, means that it is more favoured in writing in the everyday context, as at the end of the day, normal Russians...

But сильнее on google is almost 10 times more popular than сильней:

сильней:
http://www.google.co.uk/search?hl=en&sa ... 9%22&meta=

сильнее:
http://www.google.co.uk/search?hl=en&sa ......

I don't agree categorically!
It's not "best to use" in an everyday context! It's just a shortening which appears automatically in a colloquial speech! I'd almost never notice if some person said...

It's not the entirely same thing, but is similar.

Basically the normal comparative ending is -ее, but the short ending -ей can be found, mainly in poetry and older stuff, often to help make stuff...