HTC one is a quad core device, but as it has been pointed out the battery lasts about 16 hours or so... With great power comes great power consumption

And if your phone keeps dying out, i dont think it will do any business any good. I haven't heard much about galaxy S3 but I've tried out galaxy Nexus and really loved it(Even though I have an N9... I love my N9 too)

it seems to have just the right screen size. Multitasking I cannot comment on.