In my opinion you first need to decide on your overall intended use of the car. Are you wanting to jump in and take a 4-5 hour ride, or are you typically driving less than an hour to C&C and back home? Do you have a long term horsepower goal? Are you looking to set the car up to race (70-150, drag, road course) or just looking for a Dyno Queen? Next be realistic about your budget. The supporting mods for these systems can quickly out price the cost of the FI system. Anything over 650RWHP(Supercharged) and 700RWHP(TT) will require a motor build. What clutch are you going to run, transmission upgrades, differential, axles, suspension, tires, and don't forget about brakes? Are you wanting to install the kit yourself, or have it done for you? Each and every vendor is going to offer you something different in their package. When you are looking at the price, make sure you compare apples to apples. What are the materials used for the kit, and what gauge? Are you getting headers, or are the O.E.M. ones being reused? What about exhaust? The turbos themselves can range in price from a few hundred dollars to a few thousand. Are they sized for the type of spool you want to have? Are they oil and water cooled? Are you getting all the lines in your price and what are the quality of those lines? Most of the Twin turbo systems utilize a low side mount, so are you getting a quality scavenge pump system? What level of quality and airflow are you getting with the intercooler? What about heat coating? What about spring pressure for your base wastegate setting? Are you getting a boost controller? What is included in the fuel system (pumps, hanger, lines, filters, rails, injectors) and is it actually capable of supplying all the fuel you need? What is the quality of the fuel lines that are being used? Is is steel braided rubber ($6/ft) or steel braided PTFE ($20/ft)? Are you having to cut out the rear tub of your car to access the fuel tank? Are there any other parts than need to be changed because of the routing of their system or location of their parts? Something as simple as a fuel rail change can require the need for a new bracket for your throttle cable. What about tuning? SCT, VEC, Split Second, AEM, Pectel, Motec and many other options exist that give you various options and levels of performance. What about a warranty on the system? How many cars have they built and run this system? Is there any resale value to the car having a name badge from one vendor over another?
Hopefully this will help give you some questions to really consider in making your overall decision.