Since you mention you live in Georgia, I would first want to recommend that you ask your local vet if ivermectin resistant heartworm is in your area. If it is, neither of these products are reccomended. Trifexis costs a lot more but as of this time no heartworm microfilaria have built up a resistance to this drug.
If you are going to use the ivermectin in your area, the generic ivermectin and the sheep drench contain the same drug. The sheep product is easier to use since it is not as concentrated, so I would suggest you use that. A bottle will last you several years, and if you share it with a neighbor you can use the doseages that I refer to in the article.
The smaller dog will need to get 0.4 cc and the larger dog 0.5 cc. This dose is small and should be given with a tb syringe (1cc).
If you buy a bottle of the ivermectin for cattle, (the generic product) even a large dog like the Lab only needs 0.024 cc. This is less than 0.05 cc, which is really the smallest dose that you can actually give.
You would need to make up a 1/30 solution to use the cattle strength ivermectin. To do so, add 1cc of ivermectin to 30 cc of olive oil. Mix thoroughly. At this dilution the Lab will need 0.7cc, the Vizsla 0.5 cc. This one ml of ivermectin will last you about 2 years.