Same principles apply to protecting your lower extremities as the upper: you need impact resistance in the form of 'armour' at hips and knees, abrasion resistance to one degree or the other, weather protection, and ease of use (getting them on and off).
From there the fun starts! Leather gives the best crash and abrasion protection (and I'm talking full coverage here, NOT chaps) but is less comfortable on hot days, and non waterproof. Textile is a good all round material, lacking in abrasion resistance. You pays your money....
Personally I use textile (Teknik) for rides which may be inclement and leather (First Gear) for the rest.