The O/U SHOULD be more reliable than the pump or auto. Unless you are a dedicated shooter you will never know the difference. The top autoloaders on the market are VERY reliable.
The O/U will last longer than a typical repeater, but are you really going to shoot in excess of 250,000 rounds through the gun to find the advantage?
Balance of the top level autoloaders is on par with that of most sub $2000 O/U guns.
The O/U gives you the choice of choke for the barrels, with a repeater you are stuck with what is in the barrel. With the O/U you can have a light and a tight choke to work any shot that may present itself while hunting or on targets with one close and one out further. With the single barrel you have to use the tightest choke you might need on the station or hunt. Not a big deal, but still a valid point.
There is one HUGE advantage to the O/U for the target shooter, and ALL of us will find it shooting trap doubles, skeet and sporting. That is you do not have to police up hulls, and LOTS of clubs have a rule that if it hits the ground they own it keeping you from saving hulls. If you shoot more than just casually and are not independently wealthy you will end up reloading sooner or later.
There is one HUGE advantage to the autoloader and that is lower felt recoil. With lower felt recoil you can shoot longer while practicing meaningfully. If you want to get better the lowered pounding will let you take more quality shots and you can improve quicker. Light loads help in the fixed breech guns, but they still hit harder on your end of the gun.
I have several of both. I use the autos more these days, but still LOVE my O/U guns. If I am seriously going to try to hit them all or am shooting over a wager the O/U comes out simply because I have somewhere around 85-90K rounds through one in particular. It is as natural as my left arm, and I shoot it like it is. Otherwise the auto is the ticket, I shoot them pretty well and the lowered recoil is nice. Buy one of each, or pick the one you want today. Times and tastes change, if you get into shooting you will be buying a few more guns and working through them until you find "THE ONE".
Dave speaks the absolute truth. The gun does not make the shooter, it is the loose nut on the stock that makes the difference. Shoot and shoot and shoot, that is how you will get good regardless of money spent on the gun.