I have strong opinions on this and feel I can offer good feedback BECAUSE I USE BOTH! There are pros and cons to each option, but frankly, if you have any type of consistency with your irons, a rangefinder is more useful. Again, I have and use both, consequently, I feel my opinion is not narrow-minded in any way.
A rangefinder is extremely accurate. A GPS is fairly accurate too, but not to the pin. The pin moves every day. The GPS distance is only to the front, middle and back edge. And, yes, most GPS models these days allow you to point to a spot on the screen to get a distance to that point, but you are only guessing at that point when you attempt to do so. (FYI, I use a Garmin G5 Approach
.) You can shoot anything you want with a rangefinder to find the exact distance from you. Yes, I want to know the pin is exactly 137 yards from my ball, but just as importantly, or maybe more so, knowing that the lip of the greenside bunker of which I need to clear to get to the pin is 122. GPS won't give you that info. A rangefinder is invaluable. More important than a 3W to me.
On the other hand, there are some instances where a GPS can be helpful. If you can't see the pin due to a hill or obstruction of some sort, then you can't shoot it. So having a general idea of how far it is based on the GPS is convenient. Usually, I will walk to a place in which I can see it, shoot it, then pace off back to my ball for a more accurate read. Another good use for GPS is measuring the true distance of your shots. Very helpful to know for realistic averages of each club. This can also be done with a rangefinder, but I find it easier with GPS, and quicker.
If you use a cart and play when conditions are wet, you will run into "cart path only" days. This can make shooting your target before you leave the cart inaccurate with a rangefinder. So, what I do, especially if my ball is on the other side of the fairway, is use my GPS "distance to" function, which will measure the spot I point to on the screen, of which I estimate that my ball is in the general vicinity of, and from there the GPS will give the distance to the center of the green. That number will give me enough information to pick two or three clubs to use before I leave the cart. I carry my rangefinder as well. When I get to the ball I shoot the pin or anything else I need to consider. Then I decide which club to use.
The battery life on most GPS units and apps is very short. Sometimes only a round or two. I get 3-4 months out of my rangefinder battery and I play 3-4 times a week.
Another thing I have noticed is that GPS sometimes loses the signal. Or, on one particular course I play, it always gives me extremely inacurate readings, or none at all. This has happened on some other courses too. Without a rangefinder in those situations, I'd be back to stepping off yardages.
Lastly, GPS is useless for practice. Using a rangefinder, I know the distance for every shot I make on the range. It's great for dialing in your distances for each club in the bag. Of course, hitting them well is even more important! Using a rangefinder has really made a difference in my practice sessions, which has translated well to my game.
There are many rounds, that I don't even bother turning on my GPS. But, I am lost without my rangefinder.
If you can only do one, I would recommend the Bushnell Tour V2 rangefinder.
Just my experienced, yet humble opinion.