The best thing to use is the Janrain widget which encapsulates all the 3rd party login options including Microsoft, Google, Yahoo, Facebook, etc.  They handle any changes in the underlying providers over time. We use it on this site and it works very well and reduces friction for registration.

And if they use the 3rd party login they can purchase formt he store and the Site Membership Pro modules as well?

even without Janrain a user must register on the site before purchasing a premium membership, so it is no different and still reduces friction in the process for new members. Site Membership Pro does not handle site registration it only grants roles.

I don't see any connection between enabling comments and using janrain other than it might be another case where you want to have a user authenticated before allowing comments.

When an unauthenticated user vists your membership page there is a prompt to either sign in or register in order to be able to purchase membership. I believe there is even a setting to show the janrain widget on the page if janrain is enabled. After registration or logging in the user is redirected back to the page where they started ie the membership page where they can then proceed to purchase premium access. Use of Janrain reduces friction in the site registration process because users don't need to create or remember any new passwords. That is its primary benefit, they can login with credentials they already know. Under the hood we do still have to populate the password field and we don't get access to the password they use to login with whatever provider they use in the Janrain widget so a random password is generated and stored in the mp_Users table upon registration but the user doesn't need to know this password to sign in with Janrain. The user could recover the password and login without using Janrain but generally it is more convenient to just click the button in the Janrain widget to login without having to keep track of passwords for different sites. So in short the main benefit of Janrain is that users don't have to remember yet another password and that reduces friction to registering on the site.

I can't remember off the top of my head whether Janrain will connect with an existing account but you might want to test it and see. ie register manually say with a google email and then logout and try logging in/registering with Janrain and the same google account and see if it attaches to the existing account or just tells you that the email is already in use on another account. If it does connect with an existing account then there should be no confusion, but if it doesn't then you may be better off not enabling it to avoid the confusion for existing users.