There's a core of hunting enthusiasts who consider double guns to be the state-of-the-art. I am one of them. When it comes to upland game bird hunting, a well-fitting, balanced, double checks all the boxes. Generally speaking, side-by-sides are the better of the two for the uplands, but for shooters with years of experience using single-barrelled rifles, semi-autos, and pumps, transitioning to U/O's are often an easier step (due to the familiar sight plane). Whether or not the transition continues on to sxs's later is an individual comfort thing.
The question of gauge depends on availability of ammunition, but 12 gauge is indisputably the most flexible. However, in the uplands heavy loads are seldom a requirement so sub-gauges are fully viable options that can have advantages. For example, older hunters such as myself can reduce overall carry weight by 1-3 lbs between a lighter gun and lighter shells. It's important to know that "heavy loads" means shells with 1 1/8 or 1 1/4 ounces of shot. It does not refer to the "drams equivalent" gunpowder charge. Granted there has to be a larger power charge to propel a greater weight of shot, but it's referred to as a heavy load because of the weight of shot and the corresponding recoil that is produced by accelerating it out of the barrels.
So, having said this, there are appropriate shells that are loaded with 3/4 up to 1.0 ounces of shot that are deadly effective in gauges from 28 ga all the way up to 12 ga. The glaring exceptions to this "effectiveness" generality are sharptail grouse and/or pheasants. These two species are surprisingly tough birds. Still, light loads remain perfectly functional, however larger shot sizes must be used to avoid leaving injured birds in the field.
To sum up: Go with side-by-sides or under/overs depending on your initial comfort level. I would recommend choosing gauge based on local availability of ammunition. And, whatever advice you get, spend some time understanding shotgun fit, if you're not conversant in the subject already.
