Why do you have to picK? How much does it cost to see a movie? $6? Just go see both of them.
Do Simpson's first, then Harry Potter for the reason that Frankenbarry has already stated.
Heres what you do...I think that Harry Potter is longer...soo...
Go see Simpsons, do some shopping or something, then go eat, then do Harry Potter.
That would be a fun day if you like shopping. I don't do much shopping, I think of what I want at home and then go get it. So basically just screw around while you're near a theatre.
Our theatre is right next to a mall and a bunch of other cool stores (Target, Academy, Walmart, etc...) So it would be great if your theatre is set up like that.