I give the Gun4ir the big thumbs up. I built mine in a Guncon2 shell. I didn't do the solenoid either, I did a lot of reading on the Gun4ir discord channel before deciding not to include a solenoid.
The Sinden and Gun4ir from all reports are as accurate as each other. Very accurate!!
To me the game changer is the Sinden requires a white border all around the screen. The Gun4ir needs 4 IR Led sensors on each screen edge. I hate the idea of having the white border. Also the Sinden needs software running in the background, The gun4ir needs no software running, all settings are stored in the gun on the Arduino.
Have a crack at building a gun4ir, its really not that hard if you can solder