My sister has United Healthcare. After her knee replacement surgery, the social worker came in and told her they had trouble trying to place her in a rehab facility due to her insurance (she wound up having rehab at home). I am on a patient advisory council at my doctor's office which consists of 5 patients, the nurse, the nurse practitioner and key office personnel. When I brought up the problem my sister had with United Healthcare, the nurse and office secretary said they had patients who experienced similar problems with that insurance.
I have Aetna. I was on their Medicare Open Access HMO plan until a month ago. Now I'm on their Medicare PPO plan which means I'll pay the same co-pay ($10) for in network and out of network doctors. I absolutely love the plan but it is through my retiree benefits (NJSHBP), so not sure if it would be too expensive to carry it privately. I guess they would both be considered MediGap plans.
Good luck with finding out what works best for you. I know it all can be very confusing. Thankfully, NJSHBP slid me seamlessly from my regular Aetna plan to my medicare plan when it was time. .