Which of these have better flow, and which would be a better match with longtubes?

 

generally the consensus is that a 3"-into-4" ypipe with a 3.5" catback (mufflex or custom) will flow about the same as a 2.5" true duals setup...either X or H.

in my opinion...the duals always sound better.

just look at the cross sectional areas of the pipes..

single 3" = 7.07 sq in
single 3.5" = 9.62 sq in
dual 2.5" = 9.82 sq in
single 4" = 12.57 sq in
dual 3" = 14.15 sq in

 

go x pipe.