from charge balance alone, you can't decide this question (if there is a multitude of options): you'll always need to take an additional look at for example X-ray structures, IR spectra , chemical behaviour and thatlike if you're in doubt.
on the other hand: if there is a multitude of options, molecules always would tend to form the most stable variety - if they can (which is the moment where kinetics come into play, too)
now, Sn(+II) isn't very hard to oxidize ( E0 Sn(+II) / Sn(+IV) + 2 e- - 0.15 V), and peroxide is a very powerfull oxidator (E0 H2O2 + 2 H+ + 2 e- / 2 H2O + 1,78 V ) , and even if there was a polymorphism (Sn+II)(O22-) / (Sn+IV)(O2-)2 , my expectation would be that the respective internal redox reaction towards (Sn+IV)(O2-)2 would take place in a split second.
So: I'm not aware of any metal in low oxidation state (when there are higher oxidation states easily available), that would form a stable peroxide, as something like that tends to self-destruct in a New York minute.
But who knows, somewhere in deep space where it's very cold...
... a "New York minute" might become long enough to last for eons, and those species might be found:
There's a whole zoo of unlikely molecules*) , nevertheless "stable" at 3 K, to be found out there...
regards
Ingo
*)
for example cyan-isocyan, di-isocyan ... just to name two of them