Borla 140236 requires that you cut off the resonator from your stock exhaust then the Borla 140236 exhaust takes the place of your exhaust from then on.
Borla 140189 takes the place of your exhaust from after your exhaust manifolds.
They both use a X-pipe incorporated into both exhausts.
I personally have the Borla 140189. It is definitely louder than what I expected. The exhaust does have two resonators and two mufflers but I am still suprised at how loud this exhaust is. It is bearable but just not what I was expecting. It sounds great upon intial startup and is loud when engine is cold and idle is high. As vehicle warms up and engine idle RPM drops the exhaust quiets down. While cruising there is some drone but is bearable. While installing I did have fitment issues. Where the front pipe(x-pipe) meets the resonators the two pieces did not fit. Pipes were approximately 1/4" off. I cutt off one of the brackets, squeezed the two pipes closer together and re-welded the bracket in place. Another fitment issue I did have is that the left muffler hung lower than the right and the tip was point down and to the left. I corrected this by cutting off the hangers and re-welding them lower on the muffler then cutting off the tip, welding on a piece of filler then re-welding the tip back on.
So far I am not too impressed with Borla. I have already given them a call and did send them photos of what I had done and photos of the discrepancies I did find in their product. I am told that they will look into it and try and resolve the issues in any future exhausts they may build for the IS350.
As far as the Borla 140236 I do not know how it sounds but am sure it is comparable the the Borla 140189 or may be slightly quieter due the fact that it retains the factory secondary Catalysts and Factory resonator.