I'm sorry, but I vote for "cancelable'. There's no obvious reason why the "l" should be doubled. I pulled a list of "*-lable" words from MoreWords, and there's only a relative handful (below) of words with the proper suffix -able, for which this doubling is done. So it strikes me as a bit of a wart. Especially note that even MoreWords shows both cancelable and cancellable.
(This list does not include all words matched by the search; I removed those that don't use the -able on the end as a suffix meaning "able to be". For example, I removed "monosyllable". The words where L-doubling occurs are bold.)
- appealable
- articulable
- assailable
- assimilable
- bailable
- billable
- bioavailable
- boilable
- calculable
- callable
- cancelable
- cancellable
- coagulable
- compellable
- concealable
- controllable
- drillable
- expellable
- fellable
- foilable
- furlable
- gelable
- gullable
- healable
- hypercoagulable
- incalculable
- inconsolable
- incontrollable
- inviolable
- irreconcilable
- irrepealable
- isolable
- labelable
- mailable
- manipulable
- millable
- noncallable
- noncancelable
- noncontrollable
- nonrecyclable
- nonrefillable
- nonsalable
- peelable
- recallable
- reconcilable
- recyclable
- reelable
- refillable
- repealable
- resalable
- resealable
- revealable
- rulable
- sailable
- salable
- scalable
- sealable
- sellable
- spallable
- spillable
- spoilable
- stealable
- tellable
- tillable
- unappealable
- unassailable
- unassimilable
- uncontrollable
- unfulfillable
- unreconcilable
- unrecyclable
- unsalable
- unscalable
- unsellable
- untillable
- willable