here some advices:
-try them out both phones on their stores before you purchase any of them.just don't listen too much from others.
-android is different from IOS. You may feel uncomfortable when using it due to the different way of implementing. People say lag after using for several months but they didn't know that android support more in multitasking.But with newest android version(4.2.2) + quad/octa-core + 2GB RAM,i doubt it will be lag on galaxy s4.
-photo of galaxy s4 is at least a bit better than photo of iphone 5.
-material?can't say more.same plastic is on galaxy s4 and premium material is on iphone 5.
-5 inch or 4 inch screen?your choice.bigger screen although is better on daily using but it may cause uncomfortable on someone.overall screen quality on both phones are great but more pixel on galaxy s4 may improve a little more detail on screen.
-many new sensors on galaxy s4 thus many new gestures for galaxy s4 which no other smartphones have.check them out "smart pause","air view","air gesture","smart scroll".They might be gimmick for someone but i would say why not having some special features?
-galaxy s4 support micro sd card which can help your phone add some extra storage.
it is really up to personal requirement and opinion.All i can say that both are really great phones.Remember to try them out!