The following is from Plumb's Veterinary Drug Handbook, 8th Ed., Plumb, 2015...
Capstar: "Nitenpyram is usually tolerated well, but serious adverse effects have been reported. Based on poast=approval adverse drug experience reporting, the following adverse events (listed in decreasing order of frequency) have been reported: Lethargy/depression, vomiting, itching, decreased appetite, diarrhea, hyperactivity, incoordination, trembling, seizures, panting ,allergic reactions including hives, vocalization, salivation, fever, and nervousness. The frequency of serious signs, including neurologic signs and death, was greater in animals under 2 pounds of body weight, less than 8 weeks of age, and/or reported to be in poor body condition."
Comfortis: "Spinosad appears to be very well tolerated in dogs...will not show any adverse effects after dosing. The following adverse reactions are possible and based on post-approval adverse drug event reporting (listed in decreasing order of frequency): vomiting, depression/lethargy, anorexia, ataxia, diarrhea, pruritis, trembling, hypersalivation and seizures."
$120/month (you should be able to find it for less) for Comfortis. $30 for every other day dosing for 12 days or $75 for every other day dosing/month.