Steel screws of the same size are stronger than titanium screws. Stainless screws are a good substitue for steel and provide corrosion resistance. Aluminum screws in a high stress area usally break in half and leave a part stuck inside your chassis usually (cause that's the really expensive part) and ruins the part.
Aluminum screws are much weaker than steel or stainless steel screws. Here are some tensile stregnths for the materials used in screws.
Grade 8 hardended and tempered screw 150,000 pounds per square inch (psi) tensile strength. (Normal screw in a Losi kit(I believe) breaks with a brittle fracture rather than bends)
316 Stainless steel ( a very common stainless alloy) 70,000 psi There are higher strength stainless steel alloys, but they are more brittle than this alloy)
Titanium alloy 6Al-4V (6%aluminum, 4%vanadium) 95,000 psi (Alloy used by Lunsford)
TiSpec alloy used in climbing equipement 65,000 psi
Generic titanium (found in cheaper screws) 35,000 psi
T6 Aluminum Alloy 42,000 psi
nice site to read about the titanium used in climbing equipment
http://www.ushba.com/tifacts.html