as always, depends on what you want it for mainly & what level of tech you are comfortable with. first thing to remember, there is no such thing as the perfect phone or OS. they each have their own niggling faults & great features. first, unless you are a wall street type forget the blackberry. i loved my BB's but i have to admit the OS is a mess & the devices themselves have become a bit shoddy with strange implementations like the "spring loaded screens" for better tactile feel,, not good.
to compare iphone4S, a device to android, an operating system, we will have to mention some android handsets to compare physical spec details. fortunatley, or not, depending on how you look at it, you have several choices of handsets to choose from with android. the top 3 for sale in the US as i write this are the no.1 selling smartphone in the world the samsung galaxy SII. the worlds first & only smartphone with a true 720p hi def display the HTC rezound & the thinnest smartphone in the world the moto razr. these phones run todays expected specs of 1.2 -1.5 ghz dual core processors with 1G of ram. 4.3 - 4.65 inch screens. iphone4S runs 1ghz single core processor with 512 mg ram with a 3.5 inch screen. the screen & camera technology of all these phones is such that they are all amazing & deciding which is better is a matter of each individuals eye as to which you may prefer. personally i prefer the larger screens because of my eyesight & my pork sausage fingers. keep in mind the physical device dimensions of the larger screened phones is similar to the smaller screened iphone4S.
while the iphone4S is outclassed in the spec department it makes up for it in its OS.
next, operating system. the simplest way i can descibe this is,, iphone4S is great for grandmas, soccer moms, office people & people who are not really comfortable with changing a computers settings or appearance. IOS is safe, easy, one size fits all, stable. however, the stability comes at a cost of limited features & functions to keep it simple, like one of those hybrid cars.
android is for people who are comfortable with computers & like to tinker, modify, experiment & play with a multitude of possibilities. of course android phones can do everything an iphone can do plus more that an iphone cannot do, but with all that power & functions comes the need to be able to understand what you are reading. iphone = soccer moms that can play solitare on the computer & android = her kid that knows how to program the computer.