Down pipes to Y Pipe or Collector?

[LagunaSecaBlue]

I am going to do a 2-1 for my M30B35, should I use a collector or a y-pipe?

Current set up.
Stock manifolds
Stock downpipes
Y pipe or Collector with O2 Bung to 2.5
2.5 Cat
2.5 Glass Pack
2.5 DTM Muffler