There is a third....
Non-flax packed Stuffing Boxes.
To answer your question:
Avoid flax. Flax is a very fibrous chopped material that is imbedded in a waxy like substance. It resembles fibrous play-dough and is packed into a traditional SB
The other material is a tightly braided rope, that may be made from graphite fiber and may or may not have PTFE. It is designed to be as close to the performance as a dripless SB. It is packed into a traditional SB and adjusted tight to eliminate the drips.
A Dripless SB is a whole different design. There is not packing nut or flange and the principle is the a watertight seal is made by a graphite disk ring against a polished SS flange. It has to be bleed of air to make it work.
There is no good thing really about flax, so avoid it.
If you have a traditional SB, use one of the newer graphite packing material with PTFE and adjust it to get just a drip or two a minute during operation (when the shaft is turning). The downside is the the packing nut has to be adjusted periodically and eventually changed.
You can change to a dripless, but bear in mind, if a dripless breaks, you have a major problem on your hand. However, the chances of this happening are extremely small. With a traditional SB, even if the nut comes all the way off, you're getting about 2 gal/min, which a good bilge pump can keep up with for a while, while you fix.
In my boat, we have the traditional SB with the newer packing.
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