I've been a customer of USAA for 30+ years. I'm a child of a USAF officer. I currently have auto, home, checking, and a small personal property policy through them.
I've always been very happy with their service, never had a problem of any kind. I have had one very minor at fault accident that was handled flawlessly.
I have never found a cheaper comparable policy from GEICO, Amica, State Farm, or any of the other big names despite shopping my policy numerous times. I basically gave up a few years ago because USAA has always been less expensive.
If I found another policy that was a little cheaper, I'd probably stay with USAA just because I don't need the hassle of switching. If another policy was a lot cheaper and from a big well known name, I'd probably double check the coverage to make sure it was comparable and then I would switch.
I'm very surprised at the price differential in the OP and suspect that there is some reason for the big difference - either USAA knows about an accident or ticket that the other insurance company doesn't see / hasn't seen, or that the policies have radically different coverage amounts or types, or one of the quotes mistyped the age of a driver and thinks they're a male under 25, or something.
So I guess YMMV.