I will admit that I'm not as familiar with Mack and Mabel as with Millie so I might be slightly biased here (and my plan is to see both), but if you'd have to pick one then I think Millie might be more interesting to see since they're doing a bit of rewriting of the book, not to mention that the creative team looks fantastic. Though if you're unfamiliar with both shows you can always try listening to them and see which score you enjoy more.
Honestly, Mack and Mabel. It's a great score, and the show rarely gets done. Learn from my mistake, and see it while you can. I missed an opportunity when it played near me.
"Ok ok ok ok ok ok ok. Have you guys heard about fidget spinners!?" ~Patti LuPone
Doug Sills KILLED the Mack & Mabel segment at Hey, Look Me Over a couple years back – I've been eagerly waiting for this revival since then. I have a feeling Millie will be milquetoast in comparison.
Mack and Mabel for sure...i once saw a L.A. Reprise version at UCLA,, with Jane Krakowski and Douglas Sills in 2000, and always wondered why this Jerry Herman musical was a flop on Broadway....