The S1/S2 mobile App requirements are the same aren’t they? - see this link:

https://support.sonos.com/s/article/4875

if your controller device operating system is only partially supported, you won’t be able to set up a Sonos device anyway.

I would just stick with the S2 App. 

You would only need to setup an S1 system if you were using a product that was S1 only compatible.

You can’t downgrade to S1 unless there’s already an S1 player in the system. Even then it’s a bit dicey. 
TBH your best option would simply be to get a more modern tablet. 

You are correct, the support requirement appear to be identical, according to Sonos.  Google and Sonos seem to disagree here.   Google won’t install the S2 app on my tablet.   It will install the S1 app.  

Perhaps it’s the space available on your device, just as a first thought?

I would suggest that it’s perhaps approaching the time to upgrade the tablet anyway, as if it’s not supported by Sonos, then it’s certainly not supported by Google either, probably for some time too …and it’s worth upgrading from the perspective of security update for your local network anyway.

What tablet, which version of Android?

Yes, it’s probably time to upgrade.   The table is a nexus7, 2nd gen.   Android 6 is what’s on it, and yes, that’s older than Sonos supports.   I know it’s old, I just figured it would be nice if it worked.   As I said, my issue is that Sonos says you can downgrade, but when you read the fine print, (which actually does not exist), it turns out that you cannot.   But that’s OK.

Does anyone have thought about which versions of the S2 softare are best?   There are 5 versions, iOS, macOS, android, fire, and window.  I have the windows and android versions.   Maybe I should look at an iPad?

That would be completely up to personal preference 

The iOS and Android apps are largely the same, minus some cosmetic adjustments at the moment, plus the addition of TruePlay on most iOS devices.

FireOS is the same as the Android platform, it’s just served through the Amazon App Store on Kindle tablets. 

There’s no de facto “best” version of the app, but you should definitely be looking for either Android or iOS for full system control since Windows and Mac controllers don’t have all the same features as their mobile counterparts.

The Windows and Mac controllers do have a better Search experience than the mobile controllers. Well, many of us think that anyway. Many of us find the “universal search” that the mobile controllers do (which searches every service at the same time) to be a right royal pita (especially on the tiny screens), and the desktop controllers (and 3rd party controllers) do not do that.