Both. Amber is a signer app, it holds your keys and vouches for you when you sing into a client, protecting the keys. Amethyst is the client you use. For me they had the least friction while offering the most protection, basically creating what everyone says nostr actually is.
Download amber and create your accounts, then download amethyst and when you go to log in use the "signer" option. It will basically walk you though

In my opinion it depends on your chosen OS
On ios, basically non. First rule of fight club is NEVER paste your nsec raw into an app. That is basically all you can do on ios. Of the ones that accept a signer, you loose the ability to receive messages meaning you need at least one more app.
On android there are more, but these will differ slightly again to their graphene alternative. And the desktop situation gets even more kooky.
For me, ditto is the only ios app that serves notifications and messages some of the time and accepts a signer app
Graphene amber and amethyst for me have the best experience. Amethyst displays everything as it should, serves messages with their own system and has a built in Tor option to match amber.

Nostr itself is frustrating. The place is sold as a uncensorable alternative to th G's like Twitter, but then builders mostly seem to build for other builders never actually fullfilling the promise. Clients will often not get the basics right stopping people like me from recommending it to friends and customers. And things are so temperamental
My hope is that builders focus on widespread adoption and start thinking like normies, starting with the basics of just making stuff work. And work outwards from there.
My advice is follow accounts that are nostr based, yes somewhat of an echo chamber but it helps to learn about projects and builds, and updates to existing apps etc. I find it very interesting that way

Quality work

I have to say of all of the extensions i am testing this is handsdown the "classiest". I dont know enough to rate it in terms of security compared to extensions like diogel but this looks the nicest, is very slick and the sidebar layout makes so much sense once you use it