ненавидеть что-нибудь во сто крат сильнее
{vs}: ненавидеть что-нибудь во сто раз сильнее
I wanted to use "во сто крат" to hyperbolically express the idea of "гораздо + comparative", but I wonder if it is considered to be an outdated expression. Is it more common nowadays to use the genitive plural "раз"?
Yes, во сто крат is bookish while в сто раз is universal (note: в, not во). If you want to go really hyperbolic, you may as well use в тысячу / миллион раз. Or сто миллионов... you get the idea. :)
There are cases where they're not interchangeable.
You can say стократно but you can't say сторазно. Also you can say многократно, but you can't say многоразно. There's a word многоразовый, which is not quite the same.
Вó сто крат is not considered outdated. Some dictionaries mark it as colloquial and some other - as bookish. It's an idiomatic set phase (meaning immeasurably), as opposed to в сто раз which is most often used as a literal expression and is also colloquially used for an exaggeration (usually, about countable things: зарабатывать я там буду в сто раз больше).
Speaking of outdated things, literal usage of во сто крат (where крат directly replaces раз and refers to numbers) can only be found in very old literature like Russian versions of Gospel:
... иное [семя] упало на добрую землю и принесло плод: одно во сто крат, а другое в шестьдесят, иное же в тридцать.
As for your examples, the first one may sound too sophisticated for everyday speech while is OK in a letter, an article, etc. The second one ( with в сто раз) is a bit informal/colloquial/spontaneous and not suitable for something written, when applied to something that can't be expressed in numbers; гораздо or намного would sound better, applied to smth. like comparative 'amount of hatred'.
In daily life we usually use “в сотню раз”, “в сто раз” and “во сто раз”.
I’ve never heard anyone use “в/во сто крат”, but I saw it in books.
I don’t know what Sergey meant by “not во”, but I often hear people say the three versions I’ve suggested.
