A few things...
First of all, look at the barrels. The O/U has 26" barrels; the 500 has 28". The 500 has a longer receiver. The O/U is not too short; it is, however, on the shorter end of the 26-30" spectrum that is common for hunting guns. The main difference between an identical gun with 26" to 30" barrels is swing speed and balance point.
Typically, a break-action gun (single or double) will be around 4" shorter than a receiver gun (pump or autoloader) with the same barrel length. My 34" single shot trap gun is a bit barrel-heavy and, as you want in trap, swings smooth and slow, but it's only a couple inches longer than my 28" autoloader. The autoloader is, however, a much "quicker" gun. That's normal. Weight distribution is totally different.
Ballistically, there's no real difference between a 26" and a 28" gun.
People prefer different lengths and different gun designs because they swing differently, balance differently, etc.
Whether you can hit anything depends on you, and those factors, not ballistics.