iphones/ios is full of >KNOWN< holes/vulurnablilites, every time a new ios ver is released, you have to wait for an exploit to be found for anything newer then a 3gs (or it's equiv itouch/ipad). The 2/3g has such large holes it's the equivilent of missing 1 side of a building.
android has no >KNOWN< exploits since 3.0, prob earlier. That's how 'noob mode' jailbreakers work. ios def has android beat as you always have to be careful with updates (vs bootloader-unlocked devices, some of us completely refuse to buy locked-only devices)
they both likely have more, but if you dont know about it it might as well not exist.
Unlocked devices like the S5 you only need to learn once and to re-root takes 5 minutes including reboots. You never have to worry about not being able to root on it and the like.
I'd imagine more android exploits will be found as ICS is available for bootloader-locked phones, HC more or less sidestepped this issue.
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Nexus 4: Stock KOT49H (4.4.2) 2013 N7: Stock KOT49H (4.4.2)
Touchpad: CM11 KOT49H (4.4.2) iTouch 5G: Stock 10B146 (6.1.2)