The main reason I chose it is because it allows you to preview the archive contents before extracting it. I would also like to know other unarchivers that has this feature.
I think that Entropy or BetterZip might be what you are looking for:
Both allow you to peek inside or drag/drop files in and out of encrypted archives without extracting.
For an archive utility I've liked Entropy. It's not free but it's similar to 7-Zip overall and has good format support including creating both 7z and RAR archives.
For your archive question, I personally prefer to use third party tools to manage archives
My favourite is Entropy: http://www.eigenlogik.com/entropy/
If you have used WinRar before, you will fell right at home with it.
As for your other question, I'm not 100% certain, but either way I would seriously recommend checking out XtraFinder, which is a free third party add-on for Finder, which adds a host of functionality.