1) You don't have to use Frontpage Extensions with anything. Not even with Frontpage.
2) I haven't tried to edit a page in either one. And I've only seen pages (well, really looked at, source code and all) made in Publisher. But when you have two programs, one made to edit webpages, and one that is not, I would say it is an easy choice: Expression Web. And I see the program is getting good reviews, even from people who share my bias towards new Microsoft programs.

(Frontpage isn't exactly dead, I'm sure you can still use it for years to come.)