Howdy Back Ralf and All:
When you ask, "Which is more beneficial?", my first thought is beneficial how? In terms of all-out performance, economy, volume, sound quality, longivity, weight, or cost? I went with a single 2" system with a Walker DynaMax turbo type muffler for several reasons; cost, lighter weight, an less prone to rust-out. If I ever get to a strip for some all-out acceleration performance, I will disconnect the exhaust system from the header outlet.
Also know that the area of a pipe increases much faster than the diameter numbers would indicate. Here's the math.
OD (in.)....ID (in.)...Area....%Increase
1.75........1.620.......2.06.....0%
2.00........1.870.......2.74.....24.8%
2.25........2.120.......3.53.....22.37% greater than 2".
2.50........2.370.......4.41.....25% greater than 2.25"
The number of bends and obstructions inside the muffler(s) are also variable factors. The same is true for the piping.
So the question is, in your case, what is most beneficial?
Aidos, David