As a general rule, use steel or titanium in stressed areas.
Such as sub-frame bolts, rearset bolts, suspension link bolts, axle and fork pinch bolts, etc.
For non-stressed areas, aluminum is fine and much lighter.
For example, mirror mount bolts, rear seat and fender mount bolts, etc.
7076-T6 aluminum is only about as strong as low-grade steel.
For light weight and strength (and corrosion resistance), titanium is your best bet, except for cost.
For strength and corrosion resistance (and less cost than Ti), go with stainless.