a) is probably unusual (but with the rise of webmail, I suspect the distinction between Reply and Reply-all is being lost to many users now), but b) is the norm, since usually a person's Sent mail is stored when they send it anyway.

If my assumption about webmail is correct (for instance, Gmail allows you to choose Reply or Reply-all when, well, replying, but hides it behind a tiny dropdown menu), it might be good to have Reply-all as 'default' - keeps discussion in the community. Although as we see it has unfortunate side effects.