This is starting to become a bit of a hoary old chestnut. Since OP has already established that both forms are and have been in widespread use for some time, what does it mean to ask "which is correct"?

In this particular case, although the NGram doesn't really make it clear, hirable really is becoming increasingly common. So by one standard, that's the spelling of the future, and is therefore arguably 'more correct'.

On the other hand, hireable was clearly the more prevalent form in the past - certainly in various linguistic subsets (in fact, it's still the preferred British usage, though that is changing even as I write). So arguably few people would seriously criticise anyone for using a more 'traditional' form.

In short, I think this question steers dangerously close to breaching FAQ guidelines which specify that questions should preferably admit of a single clear-cut answer which most competent speakers can agree on.

"Hirable" or "hireable"
I use a swept sine to measure a transfer function, multiplying the reponse by the in phase and quadrature excitation to get a complex response at each frequency. Can you suggest an enhancement to this technique to estimate coherence as well to give an estimate of measurement quality, especially at higher frequencies where the response is small.
