Steel construction is obviously the strongest, but also the heaviest. Ask yourself if you can afford the added weight of steel before adding your load. Also, do you intend to remove the luggage rack from the vehicle when not travelling - remember the height of many 4WD vehicles exceeds the height of many underground carparks. If height is an issue, how easily can you remove the luggage rack?
Some luggage racks come as one complete unit, yet others come as separate roof rack bars with the luggage tray added separately. This feature is obviously of great advantage as you can still benefit from having roof bars for everyday transportation situations, eg. carrying sporting equipment, or large/long items, when the luggage rack is disconnected.
Aluminium construction is lighter than steel and may not be capable of carrying such a heavy load as can be carried by a steel luggage tray.
Either way, check out the roof/gutter anchor point - a luggage rack is only as strong as its anchor points. Also, aluminium will not corrode like steel.
Finally, does the luggage rack have tie-off points, and/or side rails for securing loose items?
Is a luggage storage bag available in the right size to fit inside your luggage rack if desired?