+1 on both of those, but it's better if you get one, it is better to get the kind of pickup based on what you are riding on.
If it's a lake or some place without current, the GPS is the way to go, and it is easier to install. If you mainly ride on a river that has current, like we do here on the Columbia, it is worth getting the paddle wheel installed.
The GPS does what a GPS should and tracks your speed based on it's geographic position on the planet. This is generally good, but in the case of riding where there is a current, aka most rivers, the speed will not be accurate.
For example, if you are trying to ride at 20mph with a stargazer against a 5mph current, it would see the boat as going 5mph slower (which it is) and make the boat actually travel at 20mph, but with the added speed of the current against the rider the rider would essentially be pulled at 25mph.
If you did this with a paddle wheel, it would read the 5mph from the current and adjust the boats speed to around 15mph to make the overall pulling speed 20mph.
The GPS ones will get you pretty close on a river, but won't be as accurate with a current. You may have to adjust the speed a little bit depending on current conditions.
20mph with no current in one part of the river at one part of the day might be 25mph with going downstream and 15mph going upstream different parts of the day.
Then again on a lake with no current, the GPS will be the most accurate.