Since your dog does not have any symptoms, slow kill (with ivermectin) is the safest and least painful alternative. You can go to my profile page and read an article about how to do so at home.
It is definitely a less painful alternative. Not all vets will be willing to talk to you about this, and if you decide to go with big pharma/www.heartworm society.org, be sure to ask your vet to see the needle that is used to inject that arsenic into your dog.
Thank you for caring enough about your dog to ask.