Aetna has pretty good customer service in my experience. Used Cigna many years ago so can't compare as I am sure many things have changed since then
Only have experience with Aetna and Iβve never had complaints
Apparently Aetna's plan and coverage are more u.s focused and Cigna has a number of international medical plans. Apart from my first company, I've always had Aetna and didn't have any issue.
Cigna seems more lenient in reimbusement in my experience.