The Galaxy SII only runs Android 4.1. The Galaxy Nexus only runs 4.3 if you have the proper version of the phone. I doubt either of these phones will get the next big Android upgrade, if I'm buying a phone for $250 I'd like to know it will be supported for at least a year with out having to root it and install a mod to get the newest features.
I'm running 4.2.2 on my SGSII right now. I could even update it to 4.3 if I wanted to. I don't know how you can expect a manufacturer to support a two year old phone that you bought second-hand.
iOS updates are limited by device. I forget the details but that 4S doesnt have all the functionality of that 5S. You need a minimal level phone for things like Siri and other features they never bothered backporting or couldn't due to hardware limitations.
Google OS updates are almost academic at this point. Google deploys a play store service which is a root-level middle man between your phone and various google services and apps. Google updates this themselves. Google apps are not tied to the OS, like Apple's apps are. Someone with a phone on 4.1 can get the same version of Maps that I do on 4.3. Heck, you can be on gingerbread and use all the same apps practically. Google has a huge incentive to keep gingerbread level OS phones up to date with its latest apps because so many are still in circulation.
Lastly, custom firmwares like CyoagenMod deliver up to date versions of android for many phones. So, no, its not so cut and dry. Also, if you really want an iphone, then go buy one. You'll get an easier update process but have to deal with all its limitations.
Its also worth mentioning that the galaxy nexus is a phone from 2011 that just got the newest version of android and is still a pretty beefy phone. No idea if its getting the next version officially, but it will be trivial to put a 3rd party firmware on there if not. Android has always had a phone that's updated for 2 years: the Nexus line. The only exception is the custom Nexus non-GSM nexus Sprint/Verizon sold for a short time. Google no longer sells phones like these and the Nexus is strictly GSM now as it was before.
I'm sure you can get a Galaxy SII or Galaxy Nexus for that price or lower on ebay.