Honestly I have always been affraid of used BMW's and especially Audi's as I used to have one and cost a lot to fix and maintain. You are absolutely correct with reliability between the 2. However, I wouldn't be ok with Bev unless I can go 200 miles. We used to have a leaf and continually caught ourselves far too close to zero miles. Even hit turtle mode 3 times and once was is 20° f. We didn't even consider Bev this time around. We have already gone through 10 gallons of gas over 1800 miles. It's a very nice insurance to have. Especially when it's cold. Also, my parents are 120 miles away over a mountain pass. If you do go Rex, definitely get it coded. It's super easy on a smart phone. I wouldn't have bought the car if I couldn't do that.
Take all of it with a grain of salt. I'm sure my needs are different than yours. CPO is a great way to go as prices aren't all that different. Pay attention to the original sale date when calculating how much time is left on it. Carfax makes it easy for you to find on the warranty page. My 2014 was sold in June 2015.
To answer your question, CPO will allow you to not have to worry about repair costs until it expires. By then, if you have had many costly repairs get rid of it. If not then keep the car that has treated you well. We will also know more about Rex reliability by then.
All just my opinion though. And I haven't had any issues yet.