I don't know if one is better than another but usually veterinarians unless their practice is very large, will carry one or the other. Interceptor has the advantage of killing whipworms along with the heartworms prevention and hookworm and roundworm treatment that both Heartgard Plus and Interceptor have. If whipworms are a problem in your area than it may be a medication to switch to. Here is a link to whip worm info: http://www.peteducation.com/article....&articleid=778
Also both companies that make these medications are very good companies. Novartis makes Interceptor and Merial makes Heartgard Plus. We carry Heartgard Plus at my hospital because we also carry the Merial vaccines due to their high safety especially with cats and their flea and tick prevention (Frontline Plus). I also like Merial because they do a lot of rescue work and donations for dogs and cats. During hurricane Katrina they donated over a million dollars in the treatment heartworms and ticks for the rescued dogs. Also our patients LOVE the taste of the Heartgard as it is made from real beef. Also the safety is wonderful - it takes 83 tablets (83 month supply) on average to get into a toxic dose - so I always feel safe sending home a year supply with an owner.
There is also a medication called Revolution which is great because it is a topical medication that not only takes care of fleas and ticks but also heartworms, some internal parasites and mange. So the bottom line is that there are several top of the line medications to choose from.
I am not a fan of online companies due to their 'grey market' acquiring of the medications which I think is risky. So if you can get the HeartGard filled at PetSmart or another vet then great, if the choice is switching to Interceptor from your vet or HeartGard Plus from an online source, you might want to think about switching to Interceptor. Again, many pets are on this medication. Perhaps ask your vet about Interceptors guarantee. HearGard Plus/Merial will guarantee their medication. If your pet has been tested negative and ever becomes positive for any of the parasites it covers while your pet is on the medication - the company will pay for the pet's treatment in full
. I am not familiar with Interceptor's guarantee, if it is the same then that is another plus on choosing the medication. Hope that helps