I think I will stick with heartgard. Not that I see anything wrong but I just feel it has been good for us in the past and why mess with something thats been working. I also feel better knowing its been around forever. But thats me.
When I had 8-10 dogs I used generic ivermectin (I was living in Florida then). Now that I live in the mountains of New Mexico, I don't use a heartworm preventive on the 3 dogs who do not go down into the valley because we have no mosquitoes up here. Zoe is on Heartgard because she is presently with Betty Brown in east Texas. I'll have to keep her on it after she comes home because we will go to shows in places that have mosquitoes.
I don't think it's a good idea to use generic ivermectin unless you have too many dogs to make Heartgard affordable. I certainly would discuss it with my veterinarian - although selling heartworm preventives is how some vets make money.