if an app is available both on fdroid and the play store, and you already have google services installed, then it is almost always better to use the play store version. unless fdroid has changed recently, the way apps are signed is more robust and the chain of trust more resistant to manipulation for apps on the play store.
if you are asking if the selection of apps on each distributor are on the whole more secure in the way they provide their service, that’s a more complicated question i can’t answer.
it’s a nazi haven in the same way a public library computer is a nazi haven if nazis come in and use it. the library doesn’t check your ideology when you log on, and neither does simplex.