The only time I ever use Android Pay is when they have a promotion. Once in awhile they'll run a promo along the lines of "pay five times at any grocery store and get a $15 bill credit." Sure, why not?
But otherwise, Samsung Pay all the way. Magnetic secure transmission, and the dual compatibility with my Note 8 and Gear S3 makes it more utilitarian. And then there's Samsung Rewards. Generally I'm loathed to impose my tech-related preferences on anyone. Maybe it's because my wife uses an iPhone. But in this case I've got to say that you'd be a fool to choose Android Pay over Samsung Pay when given the choice. Samsung Pay offers you points that translate fairly directly into US Dollars. They are literally paying you to use their service over Android Pay. Heck, the process of trading points for Visa debit cards is even simpler than what I have to go through to redeem cash back on my Chase card (which I also get when using Samsung Pay, BTW). Alternatively you can get free Samsung products; I recently handed over 8,000 points for a fast charge battery pack, which I now use quite regularly.
If you've got an S6 or more recent Samsung phone and you're interested in any kind of mobile payment, there's absolutely no reason not to use Samsung Pay.