I would go with the longer bag myself. I am 5’10” and have the regular length WM Alpinlite. It fits about perfect but there are times I wish it was a little longer. I like to stuff clothes in the bottom of the bag so I’m not putting them on at “room” temperature. It gets pretty snug in a regular length bag.
I can't say I agree with the others. I'm 6'0" and have owned three WM bags, and still own two. All bags were 6' 0" length and I have never wished they were longer. It may be important to add that all my bags are semi-rec shaped so any clothes that I put in the bag with me fit to the side and I don't stuff them down at the foot of the bag. Perhaps if the bags were mummy shaped I would be forced to stuff my clothes in the foot box area.
Im 5-11 and went with the 6'6" for a couple of reasons. First many forums I read mentioned that if you are a side/stomach sleeper you add a couple inches to your height. Think about your toes being pointed and adding the length of your feet to your height. Second, it gives room to fully pull the hood of the bag up around my head without worrying about hitting the end of the bag with my feet. Third, I can store the next mornings underwear or socks down in the foot box and they are nice and toasty in the am. Normally I would say empty space=cold space and that is true, but when you are talking a top tier bag I have not found having a couple extra inches in length to be cold. Also, you can get some of the down booties and they will easily fill up that extra space. I would go long.
5' 11". I owned a long and went to a regular. Different WM bags, but the long left quite a bit of empty space. I don't keep water bottles in down bags myself and am generally wearing or laying on (pillow support) extra clothing though. It's also less bag to worry about coming in contact with damp tent walls.