Well first of all I think you've picked 2 of the top single turbo upgrades to choose from. Deciding on one is dependent upon your driving habits and expectations. Is this for racing or for street, if for racing, then what type drag or open track? Most everyone tends to focus on maximum horsepower at maximum RPM. Sure, at near 7000 rpm some develop 700hp, but at 4000 rpm may slowly be rising to 200hp. Where as a set-up rated at maximum 600hp is rapidly passing 400hp at 4000 rpm. How much response or drivability are you willing to sacrafice. To me 600hp or 800hp your already well past 500hp!!! Are you really going to notice the difference between 675hp and 700hp? Probably not, but you will notice when your power comes on and how long you can use it for, untill you have to upshift into another gear. The nice feature I have found with the Garret turbos is the availability of different sized and types of turbine housings which are easily interchangable. The To4R kit comes with a single-port .96A/R exhaust housing. There is the larger dual-port 1.15A/R which will give you more hp or a smaller dual-port .84A/R which will give you better response. There's even a single-port .81A/R and a dual-port .70A/R available which are all easily interchangeable for about $100 per/housing. Unlike the T- 51 the To4 offers the dual port exhaust design which allows for increased velocity with-out sacraficing flow. This responsive dual-port design only works with a well designed dual-port header, such as HKS offers. The T-51 kit has the same high HKS quality but with a new round single-port 1.00A/R turbine housing and a 6to1 round header. The newer T-51 also adds a unique secondary type inlet on the compressor which I have yet to understand but I can assure you it's no gimmick.