Recently Jitendra Vyas asked over Twitter that if users prefer fixed header or Back to top link; that prompted me to write this post. By the way, I started writing this when he tweeted but some how missed posting it; sorry about that.
Fixed Headers are very helpful for people who are mouse users, people with low vision, elderly and learning disabilities; reason is that it will be easily accessible even when user is at the bottom of the page and headier is always visible.
At the same time, back to top link is helpful to keyboard and screen reader users so that they can quickly jumb to the top of the page to use other navigational items. Specially this is useful when users are reading or interacting with lengthy pages.
So to conclude, I recommend use of both.