I will vouch for the Edladdin adapter. I got one recently. It works, it's well made, and you can't get any better than the Genesis controller (in my case, the six button controller) for the 7800 games that need two fire buttons.
What the adapter gets you is use of two, separate fire buttons on the Genesis controller, as you would with the regular 7800 controllers. Without it, only a single fire button works, like on a 2600 controller.
While the Euro pads might be interesting, and have the perk of being an Atari branded controller that is rather unique, it simply is not as good as the Genesis controller, especially when it comes to the D pad.
IMO, best use of money is the adapter. The Atari pads will cost more and are not worth it, from a functional, game playing perspective.
Also, IMO, the 7800 sticks absolutely suck. And if a game needs only one fire button, or no fire button, I prefer using the standard 2600 controller.