The best thing really is to talk to your vet. I guess you can always get a prescription for 6 months of Nexgard and fulfil this online, you should break even cost wise.
A quick skim of papers suggests that Credilio is a new kid on the block with a fair amount of pre-clincial data behind it as well as a few field studies. One using an active comparator Nexgard looking at efficacy and tolerablility. Field studies are good as they reflect real life. The data though advises not to use in dogs with a history of seizure, I think this is a pretty standard recommendation accross all flea and tick treatments.
In the data table below, (see link...) the table representing one of the field studies, looks at adverse events experienced at a rate over 1% , note here look at percentages not numbers as ratio =2:1 vs active therapy which in this case was Nexgard. There is no real difference between the two treatments.
Some vomiting was commented on in other well controlled studies, but what this looks like in terms of a percentage is not clear, certainly this is not reported significantly in the large field study where dog are on other co-committent medications.
Data on Credeliio
So if it was me personally? Well the data looks promising, but if I had a dog who had previously reacted to a parasitic preventative, then I would stick with what I know works well, and doesn't have side effects for my dog.
