It makes sense that a particular line of dogs could all have similar reactions to the same meds. I've seen that in other breeds at our hospital with various meds.

Actually Tri-heart plus is cheapest, well probably next to Iverheart, but we don't use the other one where I work. Carrying 4 different heartworm preventatives is plenty as the inventory cost is huge. We started veering to Tri-heart when Merial kept raising their prices disproportionately about 2x a year and we already had clients balking at the prices, so we gave them another option.

If Interceptor works for you and you're happy with it no need to change. We have some clients that refuse anything but Sentinal, and others same with Heartgard. I think it's more a matter of what works for the client and what the dog will take without a huge fuss. They all work as long as the owner gives it.