Beware the 20 Watt max may be quoted for a specific work
cycle (Raymarine and Simrad
quote 25 on 75 of, so if you have one of these, the realistic figure is in the 40-80 Watts range). I think the 25% is achievable only for well tracking hull
in perfect conditions.
I believe the top-notch gyro models do eat less but they are more expensive and they consume more in stand-by (due to much developed brain boxes)).
Now a windvane is a completely different story - zero electrical
and nearly silent - big advantages for an offshore
If you have the power and if you can afford a good auto then off course you have very good reasons to go for auto.
Remember to allow for apparent wind speed drop on the windgen if you do a lot of downwind sailing. Esp. in the trade
winds area where the winds are often less than 15 knots true (below 10 apparent).