The interrupt edge should be a fairly fast fall time. Using the possibly hundreds of nanoseconds transition time of an opamp output to drive into an external interrupt input is not a good idea and should be buffered. The 74HC14 is indeed a good choice for this. If PC board space is at a premium and you feel no need for the extra 5 inverters in the 'HC14 package you could consider the use of a TinyLogic gate. For example look at this Fairchild Data Sheet for the NC7S14.

If you say you cannot use this device because it is surface mount and that you are using through hole DIP packages then you may be stuck with the 74HC14. But then you already have "big" board compared to what can be built with the surface mounted parts.

Michael Karas