My understanding is that U-Bolts are designed to stretch. I looked up on e-trailer the torque specs based on U-Bolt diameter size when i bolted mine in. The harder the metal (Grade 8) the more brittle (less flexible) the bolt is. So if the u-bolts are designed to stretch it would only make sense that it is a lower grade bolt. However I have never heard of grade 2 bolt though.
Strength-wise, grade 8 is always better. They are more brittle in the sense that they have a sharper yield and relatively smaller plastic region, but the term simply means that a grade 8 failure will happen closer to it's yield while a lower grade will deform across a wider curve. Common fasteners are considered ductile (the opposite of brittle) in normal use. It's not until you get into esoteric types that they don't necessarily have a heat treatment and tempering and would be truly brittle.
All materials have an elastic, deformation (plastic) and failure region, which is illustrated on a stress-strain curve. Staying below the yield point the material remains in the elastic region and will not deform permanently. Beyond the yield you will permanently deform but not necessarily fail. The next point is the ultimate yield point, which is where increasing load beyond the yield point will cause material deformation that eventually causes it to fail.
The minimum yield for grade 5 is 92,000 psi and for grade 8 is 130,000 psi. This is all you need to know with shear (load perpendicular to the axis), the minor diameter will drive strength. However, with bolts you also get a tensile strength, which is the point where the fastener threads will fracture, e.g. fail in tension (pulling along its axis). For grade 5 tensile is 120,000 psi and for grade 8 it is 150,000 psi. This due to the fact that the threads themselves are experiencing the stress-strain.
What this means is, for a 9/16" bolt based on min yield and min tensile:
Grade 5 in shear - 18,640 lbf
Grade 5 in tension - 23,028 lbf
Grade 8 in shear - 22,610 lbf
Grade 8 in tension - 28,785 lbf
The brittleness comes into play when you badly exceed the yield and ultimate yield and don't get a plastic response but rather just straight to failure (e.g. impact loads). Grade 8 is considered no longer ductile above 180,000 psi. But true impacts are rare, maybe bumper bolts but even those now have energy absorption that reduces the shock to them.
BTW, bolts are not strictly meant to stretch in most cases. Head bolts are one that do require exact stretch (and they usually are in plastic so are not reusable). You need minimum stretch or at least a known amount to generate the clamping force on the joint. It's friction that usually holds a joint fast and not the bolt in shear. The difference in clamping from different elongation might mean that torque value you use for a grade 5 vs grade 8 might change, but generally it's insignificant.
