Nice job being so diligent about making sure your little Cavalier has the best heartworm prevention! I love your work.
It is very tricky these days to make decisions about heartworm prevention, with so many products on the market. It really comes down to what is going to work best for you personally. Some people find the annual heartworm injection is convenient because they aren’t very good at being on time with their parasite control, and are likely to forget. However, if you are someone that likes to give flea/tick/worm preventative treatments monthly and stick to the regime, then using a product (spot on/tablet/chew) that also covers heartworm may suit you better, and could potentially work out more cost effective in the long run.
There is no one “best” heartworm treatment, it is all about ensuring you use the product as it is suggested on the packet and by your vet. Another thing to note, is that you do not want to have gaps in time where your little Cavie is not covered for heartworm. Prevention is far better than the cure, particularly for this type of parasite.
I think perhaps ringing your local vet clinic to chat more about what will be best for you and your mate, as well as what other parasite control you are currently using, would be a good way to go.
I hope this has been helpful in making a decision about what product is going to be best for you.
All the very best to you and your Cavie!