Tried both over the years and not noticed much difference. Twin pipes a bit noisier and may give the illusion of going faster. 2 into 1 good for clutch access.
Did have carb trouble with 2 into 1 and goldie silencer, would not tick over reliably. Sorted by change to standard silencer.
Getting a 2 into 1 which fits is the real problem. Some of the commonly advertised 2 into 1 systems are a poor fit, the rose on the left pipe not seating squarely at the head, poor fit in the head and a slight kick up at the end of the pipe bringing the silencer too close to the pillion footrest.
Now using an ebay purchase of a nice 2 into 1 stainless made back in the 1960s by a company called TT Tubecraft.