@Fuchur84, this is a question so simplified, that it became senseless. I can think of many such questions, e.g. “Which is the best country to live in, if you are forced to emigrate?”
The properties of these three Stores for SailfishOS-native software are quite different, also under the aspect of “security” (whichever criteria one summarises under this term):
Can software procured in a opaque process be deemed “more secure” than a software where you can inspect every bit in any step of the process, from its source code over the build process (including its machinery) to providing the final RPM package for download? My answer to this question is a clear “No”.
Does that make SailfishOS:Chum “more secure” than the Jolla Store? We do not know, because of the opaque process: It depends on the security measures Jolla employs, hence this assessment is fully based on guesses and then has to be weighed against a completely different security aspect (transparency / inspectability / reproducibility / traceability [i.e. always: for everyone]).
Consequently no proper assessment to answer this question can be made, and even if it would, people would likely still arrive at different conclusions.
[…] if you had to pick one to make it easy to understand?
Even that part of your question does not make sense to me, besides constructing an artificial scenario where one would be forced to provide an easy answer (which does not exist): These three Stores also differ significantly in which software they distribute.
- Jolla’s own software (their few apps and AlienDavik / AAS, EAX, T9) is solely distributed via then Jolla Store.
- Most third party apps (in a maintained and uncrippled version) are only available at either OpenRepos or SailfishOS:Chum, or both.
Consequently (as @Mister_Magister pointed out multiple times), if one does not want to run SailfishOS device(s) in the retarded way most sailors seem to do (i.e. only installing SailfishOS-native apps from the Jolla Store, plus Android apps), one has to utilise at least the Jolla Store and SailfishOS:Chum; but as some authors still only publish their software at OpenRepos, basically one has to use all three.
There is a single decision I meet based on “software security”, and believe it does not make much sense to put more considerations into that on Store level (contrary to, for a specific software): If some software is distributed by both, OpenRepos and SailfishOS:Chum, I prefer to install it from SailfishOS:Chum.
BTW, as the situation and Store landscape for APKs is completely different, I came to a quite different conclusion: Only F-Droid and Google Play Store via Aurora Store, because these are the only two APK stores which only had few times malicious software slipping through their (quite different from each other) curation process; all others either have this issue severely (APKPure, APKMirror), or massively (AFAIK all others), additionally some are themselves half- (e.g. Aptoide) or fully malicious.