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The best way to decide what setup you want to go with is to figure out how much power you want to make - and be realistic with it. Turbos work best in their efficient "island". Once you determine what you want you can pick what "tools" you will use to achieve that goal.
Another factor is to think about spool. I understand this thread is about twins and not singles but for the most part - upgrading an inline 6 is typically rather efficient with a twin scroll single. Now the reason I bring this up is because the gt28rs and 2871 are great upgrades for the s4 as it is a v6 motor. It is a bit more complicated to run an efficient single on it.
When you think spool - 6 cylinders - high(ish) compression motor - 3.0L will get a decent small A/R t4 or large t3 (twin scroll or standard scroll) spooling in a decent range. With twin 28rs (smaller than the 2871 so it will spool a LITTLE earlier) you have to remember that you will only have 1.5L and 3 cylinders of exhaust gas to spool them up. On my 2.3L 4cylinder I had a 28rs, a 3076, and a 35r throughout the years. The 28rs ispooled in the midle-ish 2k rpms. With almost a liter less and 1 less cylinder I can estimate it will spool a bit later (obviously not too much). I don't follow the s4 v6 motors much so I am not sure where they make full boost at with these setups.
Long story short - figure out your HP goals, figure out approximately where you would like to have full boost by, and choose your turbo. If you want 600whp with a fast spool, a large A/R turbine housing with twin scroll in the t3 flanged family may be a great choice. Going twin t25 flanged turbos is just as much work as going single t3 or t4. You will need two manifolds and two turbos and two downpipes. That can get just as expensive if not more expensive than going single. I am all for either - I will go custom with mine as I have been doing this stuff for years - I just haven't decided exactly what I will use for hardware in a year or so when I have the time and motivation to start the project.
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