Umarex Guantlet or Hatsan Flash, which would be more accurate?

I going to be assuming that the Flashpup is similar enough to the Flash, to chime in here.
Watching the shooting of flash pups, Gauntlet 1 and Gauntlet 2, I would say the .25 Gauntlet 2 is the best of them, that is having seen the Gauntlet 2 in both .25 and .22 , compared to the Gauntlet 1 in .22, running 3rd here. The .22 flashpups, one wood, one synthetic, the wood stocked one is better than the synthetic one. The Flashpups shoot probably as good as the gauntlet 1, but the second generation Gauntlet is, from the two I have handled, better than the Flashpup.

The bottle on the Gauntlet is an advantage, and the Gauntlet has some considerable aftermarket support, to say the least.
 
The Gauntlet is a long heavy gun that is regulated and has a bottle for a lot more shots. From the bench, or under ideal conditions, it is very easy to be accurate with a Gauntlet. If you are out and about the Flash would be a lot easier carry and easier to shoot accurately because you are not fighting the length and weight. In my examples that I have shot, they are both very accurate in and of themselves.

tldr/ bench=Gauntlet; thick woods, long day hunting=Flash
 
In my opinion I have a gauntlet 2 in .25 and a hatsan flash qe in .25 im considerably active outside and carrying around the flash all day an slinging it up and down and carrying it and all that while still being able to be accurate is what did it for me it’s very quiet too I will say the gauntlets do have that regulator though and that’s why I picked up one of those too