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Permethrin is vastly more effective than deet. It lasts for weeks, even across washes and kills basically any insect that touches the sprayed areas. Its so harmless to mammals(1) that ranchers spray it directly on cattle.

It is extremely toxic to marine life and indiscriminately kills insects including beneficial pollinating insects. Use with care.

(1) With some important exceptions, thanks to rileyphone for the reminder.

Please note that permethrin is still toxic to cats.

Indeed, so much so that if you have both a cat and a dog, you should not use the standard topical flea treatment on your dog (which contains permethrin) as enough may transfer to your cat to cause serious issues.

Permethrin and DEET have similar efficacies and the EPA says they're both safe to use. The main difference seems to be DEET is applied to the skin and permethrin is applied to clothing.

https://www.consumerreports.org/insect-repellent/how-deet-an...

My understanding is that DEET is a generally effective repellent -- but Permethrin is a highly effective insecticide and a generally effective repellent. It's also nice because you can generally treat a set of clothing with it and the treatment lasts for weeks at a time.