Drogues are much smaller, meant to reduce forward motion to a more controllable level in the case of heading down wind
under bare poles or being on the downward slope of large rollers.
A sea anchor
is meant to arrest motion almost completely, like an anchor
If you deploy a drogue
or an undersized sea anchor
off the bow, you run the risk still having too much sternway, which could end up damaging a rudder
or allowing green water
up in to your cockpit
If I were to only have one of the types of devices aboard, I would have a sea anchor, but I have a full keeled boat
that heaves-to very well. (I would typically use the sea anchor on a bridle
, 45-50 off the bow, hove-to, i.e. the Pardy technique.) If I wanted to run before the wind
and my speed was getting too high, I would stream lines from the stern, attaching each end to a stern cleat to make a giant loop, to reduce speed.
Fin keeled boats often have trouble properly heaving-to without fore-reaching. For those folks, it seems that the more prudent technique is to deploy a drogue and run before the wind and forget the idea of a sea anchor all together. For this purpose I think the Jordan Series Drogue is the best, or so I've heard.