Either of them will do the job, but I would probably go with the HTC One simply because of the build quality. Its solid metal rather then the plastic of samsungs

Also, even better for development, both the S4 and the One have google play editions available without the horrid touchwiz or not too bad sense 5. You also get the software updates before the standard edition phones.

HTC one play edition:
https://play.google.com/store/device...ils?id=htc_one
S4 play edition:
https://play.google.com/store/device...sung_galaxy_s4