My understanding of the differences is that the Steeda kit doesn't require drilling anything out. It goes through the existing threads and has a nut on the back side. The Lethal kit requires drilling out some threads. I'm leaning towards the Lethal kit as the shear force will be on the shank of the bolts, while the Steeda kit will still have shear forces on a threaded section.
The Steeda bolts are made in house, heat-treated and nickel-plated - rather than permanently modifying your subframe by drilling out the bushings and differential housing, Steeda designed a custom high strength hardened bolt that completely threads through the system.
Using heat-treated 4130 Chromoly steel, these custom bolts are as strong as a class 12.9 fastener and have been proven to last on some of the quickest and fastest S550 Mustangs out there, including Steeda's Silver Bullet.