Glass all the way. Blow the dust off right before sandwiching. Done. Then you have the same number of exposed surfaces (2). And if dust settles on the glass it may be out of focus, which can help.
Note the type of enlarger makes a difference. Depending on the type of system, a significant amount of heat can build up at the negative stage - particularly if you're doing a lot of burning and dodging. The heat will tend to warp the negative in an upward direction. So with smaller negatives like 35mm you're more likely to get upward deformation rather than sagging. This is why particularly with smaller negatives flatness in the carrier can be substantially improved by just using glass on top of the negative. For those worried about dust that means less surfaces to clean. I would recommend this regardless of magnification. Flatness is critical.