You don't really have a choice right now. Novartis has been saying "it's coming back soon" for over a year now. Our last report (I work at a veterinary clinic) is that they are shipping within the month (but who knows???). Sentinel is being shipped out first, and to "win back" customers who have defected to Trifexis for the added flea treatment, it is going to sell for the same price as Interceptor. JMHO- Trifexis is the superior product. It kills fleas, not just stop their reproduction.
Heartgard has been around a long time. It's a good product. Heartgard has always guaranteed the effectivness in their product and will pay for the treatment IF your dog ever tests positive. It also has a new guarantee for whipworms. Their product is not labled for it, but if your buy it from your veterinarian and your dog tests positive, they will pay for the deworming. This is again to compete with Trifexis. Collies have been reported to have sensitivity to ivermectin, but that's it that I am aware of. In 20 years I've seen one case of ivermectin reaction and that was a dog being treated for demodex, not the super low doses in Heartgard.
Your only other choices if you want to avoid ivermectin is to treat with one of the 2 topical medications. Revolution or Advantage Multi. These are also combined flea/hearworm products and are more expensive as well.
I personally always used Interceptor myself, but have switched to Trifexis and am very happy with it. My dogs picked up fleas despite using Frontline.
regards