Tyvek. Particularly with paper based framing.
Tyvek is my standard, and if the customers ask, I tell customers:
First, Tyvek acts as a moisture barrier, just as it does in the construction of new houses (but it is not moisture proof). Home construction codes require a moisture barrier, and while picture framing codes don't, it should still be a consideration. This is particularly important if the framed artwork will ever placed on an outside wall.
Second, it looks nicer, but I realize that most people won't see it as it is next to the wall. However, it will not rub off onto the wall as some colored dust covers have been known to do, in the past.
Third, a Tyvek dust cover that has been applied with a fabric glue is much easier to remove [than paper and ATG], in the unlikely event you ever want to open the frame package. For example, in a few years you decide you want different colored mats, or , if the frame is ever dropped and the art shifts because of the drop, or the glass breaks, the dust cover easily pulls off, instead of needing a razor blade and water to try to scrape the ATG off.
4th, Tyvek is not made from wood pulp, such as many dust covers are, so is better from a preservation standpoint.
And when the difference between Tyvek and paper based dust covers is only pennies a square inch, the benefit of using Tyvek is clear.