Singlet turbo, but with a twin scroll turbine housing. This gives you efficiency of a single turbo (more efficient than twin), with the benefits of a twin turbo, where the exhaust pulses
between the banks are seperated. With the twin scroll the two banks are seperated untill the exhaust gases reach the down pipe. To take it one step further, the OEM and aftermarket headers have equal length primaries, and this creates the desired scavenging effect, because each combution pulse is seperated from the next. Something a log style manifold simply can't do, due to the design and space constrictions.
The twin scroll will also outspool an open inlet turbine housing every time. This gives it a great advantage over the top mount open housing turbo kits as well.
The only way to avoid pulse interferance (increase efficiency) with a twin turbo is to build a tubular manifold where you have equal length primaries feeding the turbo. Here is a perfect example...R35 kits are now being build with tubular manifold, going away from the log style in order to reach the power levels not seen before:
http://www.extremeturbosystems.com/i...it-2-01-01.jpg
Greddy has also switched from the cast/log manifold and gone to tubular...for the same reason.
So that is one of the big advantages...efficieny.
Price is the other big factor. For starters you will save about $500 with the twin scroll single kit that is capable of producing the same power as a Stage II twin turbo kit. On top of that the twin scroll single kit comes with a complete oil cooler kit ($400 retail) value, larger fuel pump 340lph (vs 255) larger injectors 750cc (vs 600cc) as well as an oil pan spacer ($100) to increase the oil volume by 1qt. So that alone will add up to another $1,000 or so. Some nice clutch/flywheel money.
Simplicity is another thing to consider. No need to pull the engine, get a new washer fulid reserovoir, and removal of many OEM components is minimal. Car can be returned back to stock in a weekend, without ever knowing there was a turbo kit installed. Because of the "bolt on" simpicity the install cost is much less, $1,200 on the single kit, and about $2,500-$3,000 with any twin kit mainly due to the need for engine removal.
For results, you can go the the first post on the first page. There is a dyno sheet posted there.